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The Effect on Room Acoustical Parameters Using a Combination of Absorbers and Diffusers—An Experimental Study in a Classroom

The Effect on Room Acoustical Parameters Using a Combination of Absorbers and Diffusers—An... acoustics Article The E ect on Room Acoustical Parameters Using a Combination of Absorbers and Di users—An Experimental Study in a Classroom 1 , 2 1 2 Emma Arvidsson * , Erling Nilsson , Delphine Bard Hagberg and Ola J. I. Karlsson Engineering Acoustics, Lund University, John Ericssons väg 1, 221 00 Lund, Sweden; Delphine.Bard@construction.lth.se Saint-Gobain Ecophon AB, Yttervägen 1, 265 75 Hyllinge, Sweden; erling.nilsson@ecophon.se (E.N.); ola.karlsson@ecophon.se (O.J.I.K.) * Correspondence: emma.arvidsson@construction.lth.se Received: 9 June 2020; Accepted: 2 July 2020; Published: 4 July 2020 Abstract: Several room acoustic parameters have to be considered in ordinary public rooms, such as oces and classrooms, in order to present the actual conditions, thus increasing demands on the acoustic treatment. The most common acoustical treatment in ordinary rooms is a suspended absorbent ceiling. Due to the non-uniform distribution of the absorbent material, the classical di use field assumption is not fulfilled in such cases. Further, the sound scattering e ect of non-absorbing objects such as furniture are considerable in these types of rooms. Even the directional characteristic of the sound scattering objects are of importance. The sound decay curve in rooms with absorbent ceilings often demonstrate a double slope. Thus, it is not possible to use reverberation time as room parameter as a representative standalone acoustic measure. An evaluation that captures the true room acoustical conditions therefore needs supplementary parameters. The aim of this experimental study is to show how various acoustical treatments a ect reverberation time T , speech clarity C 20 50 and sound strength G. The experiment was performed in a mock-up of a classroom. The results demonstrated how absorbers, di users and scattering objects influence room acoustical parameters. It is shown that to some extent the parameters can be adjusted individually by using di erent treatments or combination of treatments. This allows for the fine-tuning of the acoustical conditions, in order to fulfill the requirements for achieving a high-quality sound environment. Keywords: room acoustics; sound di usion; sound absorption; sound scattering; sound strength; speech clarity; reverberation time 1. Introduction In ordinary public rooms, the typical acoustic treatment is a suspended absorbent ceiling. Examples of ordinary public rooms are classrooms, oces, health care premises and restaurants. Many people spend their working days in those spaces performing a variety of di erent activities. The acoustical conditions are crucial for people’s wellbeing and while supporting their activities. 1.1. Room Acoustic Parameters in Classrooms The importance of good acoustical conditions in schools, with classrooms that support speech communication, as well as concentrated work, is well documented. Several surveys in school environments have emphasized the detrimental e ects of insucient acoustic treatment in classrooms [1,2]. The e ect on cognitive functions, such as working memory, have been investigated [3], as well as on academic attainment [4]. Acoustical treatment in classrooms should not only secure good Acoustics 2020, 2, 505–523; doi:10.3390/acoustics2030027 www.mdpi.com/journal/acoustics Acoustics 2020, 2 506 listening conditions; it has been reported that teachers su er from voice disorders to a greater degree than the rest of the population [5]. Objective measures for voice support in medium-sized classrooms have therefore been developed [6–8]. The most common way to specify room acoustical target values in standards and regulations is to use reverberation time T [9]. The reverberation time is often evaluated as T , i.e., evaluating the 60 20 range5 to25 dB of the decay curve. Validation is defined in ISO 3382-2 [10]. Due to the procedure for evaluation of T , early reflections are ignored. As stated in textbooks on acoustics [11,12], it is known that two rooms with the same reverberation time can still be perceived as di erent, and that the reverberation is not solely enough to characterize room acoustical conditions. The fact that early reflections are ignored in evaluating T is a plausible explanation for the occasionally bad correlation between the perceived condition. Furthermore, in rooms with ceiling treatment, the decay curve is often double-sloped, with a steep slope at the start of the decay and a less steep slope towards the end of the decay [13]. Thus, there is an ambiguity in the evaluation of the reverberation time, due to the non-linear behavior of the decay curve. Consequently, complementary room acoustic parameters are needed, in order to capture the subjective experience of the acoustical conditions. Lochner and Burger [14] emphasized the importance of early reflections for the subjective impression of an auditorium, stating that it is the sound field and pattern of reflections that will a ect how the sound environment is experienced, rather than one single parameter. The early reflections will contribute to the direct sound and thereby to the clarity of speech. The importance of parameters including early reflections, such as speech clarity C , have also been investigated in several studies [15–20]. Another parameter for speech intelligibility is the Speech Transmission Index (STI). The parameters C and STI have been introduced in some national standards [21,22]. STI is defined in IEC60268-16 [23] and C in ISO 3382-1 [24]. C is an energy ratio for early-to-late 50 50 arriving energy expressed in dB. The time limit between early and late energy is set at 50 ms for speech. Another parameter describing the relation between early and late reflections is definition, D which is 50, expressed as a percentage. These two parameters, C and D , are exactly related. In this study C 50 50 50 has been investigated. In a recent study [25], the reading speed for Italian second graders was investigated. The study indicates a relation between reading speed and C . No correlation to reverberation time was identified. Bradley et al. [26] investigated speech intelligibility in classrooms, examining the relation between signal-to-noise ratio and room acoustic parameters. The results from [26] show that the e ect on signal-to-noise ratio is very important for speech intelligibility, and useful-to-detrimental ratios are proposed and recommended, instead of only focusing on the reverberation time. Further, they concluded that an increase in early reflections could improve signal-to-noise ratio by up to 9 dB [27]. The non-linear decay curve in a room with absorbent ceiling treatment also implies that there is a di erence in the character of the sound field at steady-state and during the latter part of the decay. It is therefore also of interest to use the parameter sound strength G defined in ISO 3382-1. This parameter is measured during steady state, and relates to how sound reflections in a room contribute to the sound pressure level. Sound strength has mainly been used for concert halls and other performance spaces [28,29], but also for the evaluation of acoustical conditions in classrooms [15]. 1.2. Room Acoustic Treatment in Classrooms The most common acoustic treatment in ordinary public rooms is a sound absorbent ceiling and traditionally, as mentioned above, the required target values are defined by the reverberation time. When most of the absorption in ordinary rooms is predominately located at the ceiling, the scattering properties of furniture and other interior equipment will a ect the acoustical conditions [30]. In fact, even the directional characteristics of sound scattering objects in a room with a suspended absorbent ceiling will be significant. If the sound scattering objects redirect the energy up onto the absorbent ceiling or in a direction towards other sound reflecting surfaces, such as walls, the outcome will be Acoustics 2020, 2 507 di erent. This circumstance also presents the possibility of using sound scattering objects in order to fine tune acoustical conditions. Di users have long been used and applied in concert halls and studios [31]. The purpose of di users is to avoid flutter echoes and to decrease the grazing sound field, but this type of treatment can also be used to direct the sound in preferable directions [32]. Absorbers reduce echoes but also decrease the sound energy levels, which can be negative in rooms such as classrooms. Acoustics 2020, 3 FOR PEER REVIEW   3  The influence of the location of sound absorbing materials on room acoustical parameters has been studied by Cucharero et al. [32]. The e ect on reverberation time, speech clarity and STI of studios [312]. The purpose of diffusers is to avoid flutter echoes and to decrease the grazing sound  sound absorbing material, and its placements in educational rooms, was calculated in a study by field, but this type of treatment can also be used to direct the sound in preferable directions [32].  Berardi et al. [33]. Choi has studied the combination of di users and absorbers on a 1/10 scale, testing Absorbers reduce echoes but also decrease the sound energy levels, which can be negative in rooms  such as classrooms.  di erent placements of absorbers and di users [34,35]. Evaluating the room acoustic parameters of The influence of the location of sound absorbing materials on room acoustical parameters has  reverberation time, speech clarity and sound strength showed that a combination of those two di erent been studied by Cucharero et al. [32]. The effect on reverberation time, speech clarity and STI of sound  types of acoustic treatment was the most preferable when considering several acoustic parameters. absorbing material, and its placements in educational rooms, was calculated in a study by Berardi et  Another aspect of classroom acoustics is the absorption of low frequencies. Listening tests show al. [33]. Choi has studied the combination of diffusers and absorbers on a 1/10 scale, testing different  placements  of  absorbers  and  diffusers  [34,35].  Evaluating  the  room  acoustic  parameters  of  preferences for configurations with good low frequency absorption, especially in cases where a high reverberation  time,  speech  clarity  and  sound  strength  showed  that  a  combination  of  those  two  ratio of low frequency sound is emitted [36]. different  types  of  acoustic  treatment  was  the  most  preferable  when  considering  several  acoustic  parameters.  1.3. Study Objective and Principal Conclusion Another aspect of classroom acoustics is the absorption of low frequencies. Listening tests show  preferences for configurations with good low frequency absorption, especially in cases where a high  The findings in the references cited above imply that the acoustic treatment in a room needs ratio of low frequency sound is emitted [36].  to deal with di erent acoustics parameters in order to achieve good acoustic quality, both for the speaker and1.3. listener  Study Objectiv . The objective e and Principal of Conclusi this study on  is to investigate the e ect of di erent types of acoustic treatment on several room acoustic parameters. The investigation was made as a series of experiments The findings in the references cited above imply that the acoustic treatment in a room needs to  deal  with  different  acoustics  parameters  in  order  to  achieve  good  acoustic  quality,  both  for  the  in a mock-up of a classroom. Configurations with porous absorbers and di users both in the ceilings speaker and listener. The objective of this study is to investigate the effect of different types of acoustic  and on the walls have been evaluated. Combinations of resonant absorbers and di users were also treatment  on  several  room  acoustic  parameters.  The  investigation  was  made  as  a  series  of  tested, in order to further investigate the possibility of improving classroom acoustics. The e ect on experiments in a mock‐up of a classroom. Configurations with porous absorbers and diffusers both  the room acoustic parameters T , C and G is evaluated. in  the  ceilings  and  on  the 20  walls 50  have  been  evaluated.  Combinations  of  resonant  absorbers  and  diffusers  were  also  tested,  in  order  to  further  investigate  the  possibility  of  improving  classroom  2. Materials acou and stics. Methods  The effect on the room acoustic parameters T20, C50 and G is evaluated.  2. Materials and Methods  2.1. Room Mock-Up 2.1. Room Mock‐Up  The experiments were conducted in a mock-up of a classroom with dimensions 7.32 m 7.57 m 3.5 m. The ceiling The covers experime 7.2 ntsm  were  7.2 cond mucted and in was  a mo installed ck‐up of a at classroom height wi 2.70 th dim m.eDimensions nsions 7.32 m × and 7.57 m coor   dinate ×  3.5  m.  The  ceiling  covers  7.2  m  ×  7.2  m  and  was  installed  at  height  2.70  m.  Dimensions  and  system are shown in Figure 1. The room has a concrete sot, linoleum floor and gypsum walls. coordinate system are shown in Figure 1. The room has a concrete soffit, linoleum floor and gypsum  One of the walls represents a facade with three windows included. There are doors on the other walls, walls. One of the walls represents a facade with three windows included. There are doors on the other  see Figure 2. walls, see Figure 2.  Figure 1. Dimensions of room used in the experiments. Coordinate system where x is the width, y is Figure 1. Dimensions of room used in the experiments. Coordinate system where x is the width, y is  the length and z is the height of the room. the length and z is the height of the room.  Acoustics 2020, 2 508 Acoustics 2020, 3 FOR PEER REVIEW   4  Figure 2. The classroom with furniture, (a) from the back, (b) from the corner, (c) from the front, Figure 2. The classroom with furniture, (a) from the back, (b) from the corner, (c) from the front, (d)  (dfrom ) from  thethe  upper upper  corner, corner  inclu , including ding coordinates coordinates  for the for room. the  room. The room of 55 m  was furnished with 11 tables and 18 slightly upholstered chairs, but no other  The room of 55 m was furnished with 11 tables and 18 slightly upholstered chairs, but no other furniture. The room was equipped with a whiteboard, a flip board screen and luminaires on the walls  furniture. The room was equipped with a whiteboard, a flip board screen and luminaires on the walls (see Figure 2).  (see Figure 2). 2.2. Room Acoustic Parameters and Measurements  2.2. Room Acoustic Parameters and Measurements The  room acoustic  parameters  evaluated  are sound  strength (G)  Equation  (1),  speech  clarity  The room acoustic parameters evaluated are sound strength (G) Equation (1), speech clarity (C50), Equation (2) and reverberation time (T20). Measurements were performed using the DIRAC  (C ), Equation (2) and reverberation time (T ). Measurements were performed using the DIRAC 50 20 system (DIRAC type 7841, v.6.0). G was measured using a constant sound power source placed on  system (DIRAC type 7841, v.6.0). G was measured using a constant sound power source placed on the the floor. An exponential sweep signal was used as excitation for evaluation of C50 and T20. In the  floor. An exponential sweep signal was used as excitation for evaluation of C and T . In the latter, latter, an omnidirectional loudspeaker with dodecahedron geometry was used. The 50 center20 of  the  an omnidirectional loudspeaker with dodecahedron geometry was used. The center of the loudspeaker loudspeaker was at 1.55 m from the floor. An omnidirectional microphone was used as a receiver at  1.20 m from the floor. Two source positions and six receiver positions have been used; for positions  was at 1.55 m from the floor. An omnidirectional microphone was used as a receiver at 1.20 m from the see Figure 3.  floor. Two source positions and six receiver positions have been used; for positions see Figure 3. Sound strength G is defined as  Sound strength G is defined as ( ) h t dt ℎ 𝑡 𝑑𝑡 G = (1) dir 𝐺 2 (1)  h (t)dt 10m 0ms ℎ 𝑡 𝑑𝑡 Speech clarity C is defined as 50ms Speech clarity C50 is defined as  h (t)dt C = R (2) h (t)dt 50ms where h(t) is the impulse response; h is the impulse response at 10 m in a free field. 10m Acoustics 2020, 3 FOR PEER REVIEW   5  ℎ 𝑡 𝑑𝑡 (2)  ℎ 𝑡 𝑑𝑡 Acoustics 2020, 2 509 where h(t) is the impulse response; h10m is the impulse response at 10 m in a free field.  Figure 3. Room dimensions, source positions S1–S2 and receiver positions R1–R6. Figure 3. Room dimensions, source positions S1–S2 and receiver positions R1–R6.  In both speech clarity and sound strength, the early reflections are included. When evaluating T according to ISO 3382-2, the evaluation interval is 5 to 25 dB, given that the early reflections In both speech clarity and sound strength, the early reflections are included. When evaluating  20, are excluded. The evaluation concerns octave bands in range 125–4000 Hz averaged over source and T20, according to ISO 3382‐2, the evaluation interval is −5 to −25 dB, given that the early reflections are  microphone positions. excluded. The evaluation concerns octave bands in range 125–4000 Hz averaged over source and  The measurements were performed over the course of two days, with stable temperature microphone positions.  and humidity conditions. It was secured that there was no influence of background noise in The measurements were performed over the course of two days, with stable temperature and  the measurements. humidity  conditions.  It  was  secured  that  there  was  no  influence  of  background  noise  in  the  2.3. Repeatability Test of Measurement Method measurements.  A repeatability test for the measurement procedure used was performed. Impulse response measurements in the classroom mock-up, shown in Figure 2 were repeated five times. The room 2.3. Repeatability Test of Measurement Method  was furnished and had a suspended absorbent a ceiling. The practical absorption coecients for the ceiling are shown in Figure 4. Between each measurement, the equipment i.e., the loudspeaker A  repeatability  test  for  the  measurement  procedure  used  was  performed.  Impulse  response  and the microphone, was taken out from the room and reinstalled at di erent positions. Further, measurements in the classroom mock‐up, shown in Figure 2 were repeated five times. The room was  the loudspeaker was rotated, as this too can influence the measurements [37]. The measurement was furnished  and  had  a  suspended  absorbent  a  ceiling.  The  practical  absorption  coefficients  for  the  performed during the course of one day. Temperature and humidity were kept stable during the ceiling are shown in Figure 4. Between each measurement, the equipment i.e., the loudspeaker and  measurement procedures. However, with regards to the loudspeaker, it was always located at the front the microphone, was taken out from the room and reinstalled at different positions. Further, the  of the room in the vicinity of the teacher ’s desk. Two loudspeaker positions and six receiver positions for each loudspeaker position were used. Thus, a total of twelve observations were collected for each loudspeaker was rotated, as this too can influence the measurements [37]. The measurement was  measurement. The loudspeaker and microphone were always at least one meter from the surrounding performed during the course of one day. Temperature and humidity were kept stable during the  walls, and the receiver positions were no less than two meters from the loudspeaker. measurement procedures. However, with regards to the loudspeaker, it was always located at the  The purpose of the repeatability test was to establish the variation in the averaged room acoustical front of the room in the vicinity of the teacher’s desk. Two loudspeaker positions and six receiver  parameters reverberation time T , speech clarity C and sound strength G, when averaged over the 20 50 positions for each loudspeaker position were used. Thus, a total of twelve observations were collected  twelve combinations of loudspeaker and receiver positions. Knowing this variation gives an indication of the measurement procedure’s influence when comparing di erent scenarios of acoustical treatment for each measurement. The loudspeaker and microphone were always at least one meter from the  and the significance of the results. The spatial variation over positions, see Appendix A, are naturally surrounding walls, and the receiver positions were no less than two meters from the loudspeaker.  much larger than the variation of the averaged values in the repeatability test. It can, however, be noted Acoustics 2020, 2 510 that the spatial variation in rooms with ceiling treatment probably di er from what is expected under di use field conditions [38]. The repeatability test was performed for the basic configuration, i.e., the classroom with the furniture and ceiling treatment in Figure 4. The results of the five measurements of T , C and G are 20 50 presented in Table 1. The standard deviation for each pair of possible combinations, i.e., ten di erent combinations, from the five measurements was calculated and averaged. These results are also shown in Table 1 with the relative standard deviation is presented in the last column. Assuming normal distribution, an approximate uncertainty limit corresponding to a 95% confidence interval is presented in Table 2. Table 1. Results of repeatability test of the measurement method used in the study. Average values over the five di erent measurements, columns 2–6, average for each octave, column 7 and relative standard deviation, column 8. (a) Sound Strength G (dB) Test 1 2 3 4 5 G  /G ,avg avg ,avg 125 Hz 21.4 21.6 21.9 21.5 20.5 21.4 0.45 250 Hz 20.8 20.7 21.3 20.8 20.8 20.9 0.16 500 Hz 19.8 19.8 19.5 19.6 20.2 19.8 0.09 1000 Hz 19.3 19.0 19.0 19.2 19.5 19.2 0.04 2000 Hz 18.0 17.8 18.4 18.6 18.4 18.2 0.07 4000 Hz 18.0 17.7 18.5 18.2 18.4 18.2 0.05 (b) Speech Clarity C50 (dB) Test 1 2 3 4 5 C  /C 50.avg avg 50.avg 125 Hz 0.7 0.4 0.8 1.3 1.7 1.0 0.02 250 Hz 1.3 1.1 1.2 1.5 1.7 1.4 0.01 500 Hz 3.0 3.4 3.1 2,7 3.5 3.1 0.01 1000 Hz 4.6 4.4 4.0 4.4 4.2 4.3 0.01 2000 Hz 3.7 3.7 3.9 4.6 3.8 3.9 0.02 4000 Hz 5.0 4.4 5.3 5.1 5.0 5.0 0.01 (c) Reverberation Time T20 (s) Test 1 2 3 4 5 T  /T avg 20.avg 20.avg 125 Hz 1.50 1.43 1.44 1.40 1.56 1.47 0.038 250 Hz 1.45 1.43 1.47 1.44 1.45 1.45 0.009 500 Hz 0.81 0.81 0.82 0.82 0.83 0.82 0.009 1000 Hz 0.77 0.77 0.76 0.76 0.76 0.76 0.006 2000 Hz 0.91 0.89 0.89 0.91 0.90 0.90 0.008 4000 Hz 0.88 0.89 0.88 0.89 0.89 0.89 0.007 Table 2. Uncertainty interval related to repeatability, corresponding to a 95 % confidence interval, for the measurement procedure used in the experiments. G (dB) C (dB) T (s) avg 50,avg 20,avg 125 Hz 0.61 0.56 0.077 250 Hz 0.30 0.29 0.018 500 Hz 0.40 0.29 0.010 1000 Hz 0.25 0.27 0.006 2000 Hz 0.37 0.38 0.010 4000 Hz 0.36 0.36 0.008 It is concluded that the variations in repeated measurements are less than just noticeable di erences (JND), according to ISO 3382-1. This supports the discussion of significant di erences in the measurements. Acoustics 2020, 2 511 Acoustics 2020, 3 FOR PEER REVIEW   6  Figure 4. Absorption coecient for a 40 mm glass wool product used as absorbing material in the Figure 4. Absorption coefficient for a 40 mm glass wool product used as absorbing material in the  experiments. Blue: Absorption coecient for overall depth (ODS) 200. Red: ODS 50 mm. experiments. Blue: Absorption coefficient for overall depth (ODS) 200. Red: ODS 50 mm.  2.4. Acoustic Treatment The purpose of the repeatability test was to establish the variation in the averaged room acoustical  parameters reverberation time T20, speech clarity C50 and sound strength G, when averaged over the  2.4.1. Absorbing Material twelve  combinations  of  loudspeaker  and  receiver  positions.  Knowing  this  variation  gives  an  The absorbing material used in the form of ceiling panels and wall panels is a glass wool product indication  of  the  measurement  procedure’s  influence  when  comparing  different  scenarios  of  with a thickness of 40 mm and air flow resistivity of 40 kPa*s/m . The practical absorption coecient, acoustical  treatment  and  the  significance  of  the  results.  The  spatial  variation  over  positions,  see  , for the material according to ISO 11654 [39] can be seen in Figure 4 below. The absorption Appendix A, are naturally much larger than the variation of the averaged values in the repeatability  performance is shown for overall depth (ODS) 200 mm, according to specification in standard as well test. It can, however, be noted that the spatial variation in rooms with ceiling treatment probably  as for ODS of 50 mm, which represents the behavior of the material when mounted directly on the differ from what is expected under diffuse field conditions [38].  wall. This will be explained in a further section on configurations. The weighted absorption coecient The repeatability test was performed for the basic configuration, i.e., the classroom with the  is equal to 1 for both ODS set-ups. furniture and ceiling treatment in Figure 4. The results of the five measurements of T20, C50 and G are  For evaluation of the e ect of low frequency absorption, experiments were carried out with added presented in Table 1. The standard deviation for each pair of possible combinations, i.e., ten different  absorption on top of the suspended ceiling. The product used was a 50 mm glass wool product with combinations, from the five measurements was calculated and averaged. These results are also shown  air flow resistivity of 10 kPa*s/m encapsulated in a plastic foil. in Table 1 with the relative standard deviation is presented in the last column. Assuming normal  distribution,  an  approximate  uncertainty  limit  corresponding  to  a  95%  confidence  interval  is  2.4.2. Di users presented in Table 2.  The di users used were made of a wood frame with a surface of a curved hardboard. All di users It  is  concluded  that  the  variations  in  repeated  measurements  are  less  than  just  noticeable  tested had the same geometry and dimensions 600 mm 600 mm 100 mm, see Figure 5. Air gaps on differences (JND), according to ISO 3382‐1. This supports the discussion of significant differences in  the sides in combination with the enclosed volume gives the di user a Helmholtz resonance in the the measurements.  frequency range of 125–250 Hz. Acoustics 2020, 3 FOR PEER REVIEW   8  Table 1. Results of repeatability test of the measurement method used in the study. Average values  over the five different measurements, columns 2–6, average for each octave, column 7 and relative  standard deviation, column 8.  (a) Sound Strength G (dB)  Test  1  2  3  4  5  G,avg σavg/G,avg  125 Hz  21.4  21.6  21.9  21.5  20.5  21.4  0.45  250 Hz  20.8  20.7  21.3  20.8  20.8  20.9  0.16  500 Hz  19.8  19.8  19.5  19.6  20.2  19.8  0.09  1000 Hz  19.3  19.0  19.0  19.2  19.5  19.2  0.04  2000 Hz  18.0  17.8  18.4  18.6  18.4  18.2  0.07  4000 Hz  18.0  17.7  18.5  18.2  18.4  18.2  0.05  Figure 5. Sketch of diffuser used in the study, horizontally oriented.  Figure 5. Sketch of di user used in the study, horizontally oriented. (b) Speech Clarity C50 (dB)  Test  1  2  3  4  5  C50.avg σavg/C50.avg  Diffusion  characteristics  were  measured  in  a  semi‐anechoic  chamber.  The  energy  in  the  125 Hz −0.7 −0.4 −0.8 −1.3 −1.7 −1.0 −0.02  reflections were estimated from impulse responses using windowing techniques, excluding the direct  sound. The reflections were measured for azimuthal angles (θ) 0–90 degrees. Figure 6 presents the  diffusion characteristics for 500, 2000 and 4000 Hz, and the assumption of symmetrical properties has  been applied.  (a)  (b)     (c)  (d)  Figure 6. Diffusion characteristics at (b) 500, (c) 2000 and (d) 4000 Hz. The upper left figure (a) shows  the orientation of the diffusers relative room coordinates, see Figure 1.  The diffusers were tested in a vertical and a horizontal direction. In the vertical the majority of  sound waves were directed in z‐direction, Figure 7. while for horizontal is the majority of waves  directed in x‐y plane, Figure 8.  Acoustics 2020, 3 FOR PEER REVIEW   8  Figure 5. Sketch of diffuser used in the study, horizontally oriented.  Acoustics 2020, 2 512 Diffusion  characteristics  were  measured  in  a  semi‐anechoic  chamber.  The  energy  in  the  reflections were estimated from impulse responses using windowing techniques, excluding the direct  Di usion characteristics were measured in a semi-anechoic chamber. The energy in the reflections sound. The reflections were measured for azimuthal angles (θ) 0–90 degrees. Figure 6 presents the  were estimated from impulse responses using windowing techniques, excluding the direct sound. diffusion characteristics for 500, 2000 and 4000 Hz, and the assumption of symmetrical properties has  The reflections were measured for azimuthal angles () 0–90 degrees. Figure 6 presents the di usion been applied.  characteristics for 500, 2000 and 4000 Hz, and the assumption of symmetrical properties has been applied. (a)  (b)     (c)  (d)  Figure Figure  6. Diffusion 6. Di usion  characteristics characteristics atat (b (b) )500, 500, ((c c)) 2000 2000 and and( ( dd ))4000  4000 Hz.  Hz. The  The upper  upper left left figur fiegure (a) shows  (a) shows  the orientation of the di users relative room coordinates, see Figure 1. the orientation of the diffusers relative room coordinates, see Figure 1.  The di users were tested in a vertical and a horizontal direction. In the vertical the majority of The diffusers were tested in a vertical and a horizontal direction. In the vertical the majority of  sound waves were directed in z-direction, Figure 7. while for horizontal is the majority of waves sound waves were directed in z‐direction, Figure 7. while for horizontal is the majority of waves  directed in x-y plane, Figure 8. directed in x‐y plane, Figure 8.  Acoustics 2020, 2 513 Acoustics 2020, 3 FOR PEER REVIEW   9  Acoustics 2020, 3 FOR PEER REVIEW   9  Figure 7. Vertically oriented diffusers, majority of reflections will be sent in z‐direction.  Figure 7. Vertically oriented diffusers, majority of reflections will be sent in z‐direction.  Figure 7. Vertically oriented di users, majority of reflections will be sent in z-direction. Figure 8. Horizontally oriented diffusers, majority of reflections will be sent in x‐direction.  Figure 8. Horizontally oriented di users, majority of reflections will be sent in x-direction. Figure 8. Horizontally oriented diffusers, majority of reflections will be sent in x‐direction.  2.5. Configurations 2.5. Configurations  2.5. Configurations  For this experimental series, nine di erent configurations were tested, starting from the empty For this experimental series, nine different configurations were tested, starting from the empty  For this experimental series, nine different configurations were tested, starting from the empty  room. Thereafter, there was an absorbent ceiling, with properties according to Section 2.4.1. added room. Thereafter, there was an absorbent ceiling, with properties according to Section 2.4.1. added  room. Thereafter, there was an absorbent ceiling, with properties according to Section 2.4.1. added  and further was the room furnished. From this configuration was di erent type of wall treatment and further was the room furnished. From this configuration was different type of wall treatment  and further was the room furnished. From this configuration was different type of wall treatment  added, three di erent configurations: absorbing material, according to Section 2.4.1; vertically oriented added,  three  different  configurations:  absorbing  material,  according  to  Section  2.4.1;  vertically  added,  three  different  configurations:  absorbing  material,  according  to  Section  2.4.1;  vertically  di users, as in Figure 7; and horizontally oriented di users, as in Figure 8. Additional di users were oriented  diffusers,  as  in  Figure  7;  and  horizontally  oriented  diffusers,  as  in  Figure  8.  Additional  oriented  diffusers,  as  in  Figure  7;  and  horizontally  oriented  diffusers,  as  in  Figure  8.  Additional  installed in the ceiling. These di users were located in the front area of the room, i.e., in a typical diffusers were installed in the ceiling. These diffusers were located in the front area of the room, i.e.,  diffusers were installed in the ceiling. These diffusers were located in the front area of the room, i.e.,  speaker position. In the last configurations, low frequency absorption was investigated; Helmholtz in a typical speaker position. In the last configurations, low frequency absorption was investigated;  in a typical speaker position. In the last configurations, low frequency absorption was investigated;  resonance absorption by di users, compared to a porous absorber with properties for good absorption Helmholtz resonance absorption by diffusers, compared to a porous absorber with properties for  Helmholtz resonance absorption by diffusers, compared to a porous absorber with properties for  properties in this frequency range (see the last section of Section 2.4.1). All configurations are described good  absorption  properties  in  this  frequency  range  (see  the  last  section  of  Section  2.4.1).  All  good  absorption  properties  in  this  frequency  range  (see  the  last  section  of  Section  2.4.1).  All  in Table 3. For full abbreviations and definitions see Abbreviations. configurations are described in Table 3. For full abbreviations and definitions see Abbreviations.  configurations are described in Table 3. For full abbreviations and definitions see Abbreviations.  Acoustics 2020, 2 514 Table 3. Configurations in the test series: Configuration number, definition with abbreviations and description of the configurations. Configuration Configuration Definition Configuration Description 1 Empty No acoustic treatment 2 51.8CA 51.8 m absorptive ceiling 3 51.8CA_F (2) + furniture 4 51.8CA_F_8.64WA (3) + 8.6 m wall absorbers 5 51.8CA_F_8.64VWD (3) + 8.6 m vertical wall di users 6 51.8CA_F_8.64HWD (3) + 8.6 m horizontal wall di users 2 2 47.5 m absorptive ceiling, 4.3 m ceiling 7 47.5CA_4.3CD_F_ 8.6WA di users, furniture and 8.6 m wall absorbers 8 51.8CA_8.6VWD (2) + 8.64 m vertical wall di users (2) + 25.0 m low frequency absorptive 9 51.8CA_25.0LFMA mineral wool added in the ceiling 3. Results The following section is divided into four di erent subsections, presenting the room acoustic parameters for di erent configurations. Section 3.1. represents the e ect of traditional acoustic treatment and furniture. Section 3.2. includes the e ect of acoustic treatment, absorbers and di users, on the walls, using the di users in di erent orientations. Section 3.3 describes how the di users were placed on the ceiling and Section 3.4. includes the e ect of additional low frequency absorption. The results are presented in the form of diagrams, evaluated over octave frequency bands. All values for the room acoustic parameters with the corresponding standard deviation are presented in Appendix A. 3.1. E ect of Absorbent Ceiling and Furniture The graphs presented in Figure 9 show the room acoustic parameters for a room without treatment (Empty), a room with absorbent ceiling (51.8CA) and a room with absorbent ceiling and furniture (51.8CA_F). For description of furnishing see Section 2.1. Room mock-up. Comparing the empty room with a configuration using acoustic ceiling shows a clear di erence for all acoustic parameters over the entire frequency range, with the strongest change from 500 Hz and upwards, which can be correlated to the acoustic performance of the ceiling (see Figure 4). The sound strength in Figure 9a decreases by as much as 8 dB at these frequencies, and speech clarity, in Figure 9b, by 7 dB. The reverberation time, in Figure 9c, decreases to approximately half the value. The reason for the short reverberation times at low frequencies, in an already empty room, is due to the fact that the surrounding walls in the classroom were lightweight walls of plaster board. Adding furniture contributes by scattering the sound and is e ective from 500–2000 Hz, with the largest di erences at 500–1000 Hz, resulting in an additional change in curve shapes for the room acoustic parameters. The sound strength value is mainly dependent on the absorption area, but a decrease of about 1 dB for the frequencies 500–4000 Hz can still be found for this parameter, due to scattering and minor absorption from the upholstered chairs. It should be noted that 1 dB is considered a just noticeable di erence (JND) for sound strength, according to ISO 3382-1 [24]. The speech clarity and reverberation time is a ected in a more limited frequency range, 500–1000 Hz, with significant di erences. Regarding speech clarity, an increase of 3 dB is achieved at 500 Hz, and 2 dB at 1000 Hz. As for sound strength, 1 dB di erence in speech clarity is considered to be JND, according to [24]. The reverberation time decreased by nearly half at 500 Hz, from 1.5 s to 0.8 s, with less reduction at 1000 Hz and 2000 Hz, although still a noticeable di erence, a decrease of 0.6 s at 1000 Hz, and 0.3 s at 2000 Hz. JND for a reverberation time is a change of 5% [24]. Acoustics 2020, 3 FOR PEER REVIEW   11  Acoustics 2020, 3 FOR PEER REVIEW   11  Acoustics 2020, 2 515 Figure  9.  Room  acoustic  parameters,  (a)  sound  strength  (G),  (b)  speech  clarity  (C50)  and  (c)  reverberation time (T20). In blue: room with no treatment (Empty). Red: room with absorbent ceiling   Figure 9. Room acoustic parameters, (a) sound strength (G), (b) speech clarity (C ) and (c) reverberation (51.8CA). Green: room with absorbent ceiling and furniture (51.8CA_F). Installation of ceiling gives  Figure  9.  Room  acoustic  parameters,  (a)  sound  strength  (G),  (b)  speech  clarity  (C50)  and  (c)  time (T ). In blue: room with no treatment (Empty). Red: room with absorbent ceiling (51.8CA). Green: solidreverberation  differences  for time the  (T 20entir ). In eblue:  frequency  room with  range  no  while treatment  furniture  (Empty).  has Red:  highest  room eff  with icien  abso cyrbent  at 500–2000  ceiling  Hz.  room with absorbent ceiling and furniture (51.8CA_F). Installation of ceiling gives solid di erences for (51.8CA). Green: room with absorbent ceiling and furniture (51.8CA_F). Installation of ceiling gives  All parameters are affected by the furniture; thus, the greatest differences are seen for C50 and T20.  the entire frequency range while furniture has highest eciency at 500–2000 Hz. All parameters are solid differences for the entire frequency range while furniture has highest efficiency at 500–2000 Hz.  a ected by the furniture; thus, the greatest di erences are seen for C and T . 50 20 All parameters are affected by the furniture; thus, the greatest differences are seen for C50 and T20.  Comparing  the  measured  value  to  calculation  with  assumption  of  diffuse  sound  field  using  Comparing the measured value to calculation with assumption of di use sound field using Sabine’s formula shows substantially lower values for calculation over the entire frequency range  Comparing  the  measured  value  to  calculation  with  assumption  of  diffuse  sound  field  using  Sabine’s formula shows substantially lower values for calculation over the entire frequency range compared to the measurement, see Figure 10. Note also that the shape of the two curves differs. The  Sabine’s formula shows substantially lower values for calculation over the entire frequency range  compared to the measurement, see Figure 10. Note also that the shape of the two curves di ers. slight  increase  at  higher  frequencies  in  the  measured  curve  often  appears  in  sparsely  furnished  compared to the measurement, see Figure 10. Note also that the shape of the two curves differs. The  The slight increase at higher frequencies in the measured curve often appears in sparsely furnished rooms, due to the lateral reflections from walls. The frequency‐dependent effect of scattering is not  slight  increase  at  higher  frequencies  in  the  measured  curve  often  appears  in  sparsely  furnished  rooms, due to the lateral reflections from walls. The frequency-dependent e ect of scattering is not included  in  the  Sabine  calculation,  but  appears  as  a  valley  in  the  measured  curve.  Measured  rooms, due to the lateral reflections from walls. The frequency‐dependent effect of scattering is not  included in the Sabine calculation, but appears as a valley in the measured curve. Measured absorption absorption from furniture is taken into account in the calculation.  included  in  the  Sabine  calculation,  but  appears  as  a  valley  in  the  measured  curve.  Measured  from furniture is taken into account in the calculation. absorption from furniture is taken into account in the calculation.  Figure 10. In blue measured and in red calculated reverberation time for the room with absorbent  Figure 10. In blue measured and in red calculated reverberation time for the room with absorbent Figure 10. In blue measured and in red calculated reverberation time for the room with absorbent  ceiling and furniture.  ceiling and furniture. ceiling and furniture.  3.2. The E ect of Acoustic Treatments on the Walls 3.2. The Effect of Acoustic Treatments on the Walls  3.2. The Effect of Acoustic Treatments on the Walls  The following graphs in Figure 11. present the room acoustic parameters for configurations The following graphs in Figure 11. present the room acoustic parameters for configurations with  The following graphs in Figure 11. present the room acoustic parameters for configurations with  with acoustic treatment placed on the adjacent walls. Two walls are used, 4.3 2 m is covered with acoustic  treatment  placed  on  the  adjacent  walls.  Two  walls  are  used,  4.3  m   is  covered  with  the  acou the stictr  trea eatment tment on  pla each ced wall   on for theall  adjac three ent configurations   walls.  Two  pr walls esented   are in used, this  section, 4.3  m   is i.e.,  covered coverage  wit 2 ofh  the  treatment on each wall for all three configurations presented in this section, i.e., coverage of 8.6 m  in  8.6 m in total. The configurations are with wall absorption (51.8CA_F_8.6WA), vertically oriented treatment on each wall for all three configurations presented in this section, i.e., coverage of 8.6 m  in  total.  The  configurations  are  with  wall  absorption  (51.8CA_F_8.6WA),  vertically  oriented  wall  wall di users (51.8CA_F_8.6VWD) and horizontally oriented wall di users (51.8CA_F_8.6HWD) diffusers  (51.8CA_F_8.6VWD)  and  horizontally  oriented  wall  diffusers  (51.8CA_F_8.6HWD)  (see  total.  The  configurations  are  with  wall  absorption  (51.8CA_F_8.6WA),  vertically  oriented  wall  (see Figures 7 and 8). For all configurations in this section, a full covering absorbent ceiling is installed Figure  7  and  Figure  8).  For  all  configurations  in  this  section,  a  full  covering  absorbent  ceiling  is  diffusers  (51.8CA_F_8.6VWD)  and  horizontally  oriented  wall  diffusers  (51.8CA_F_8.6HWD)  (see  and the room is sparsely furnished, as in configuration 51.8CA_F, which is also included in graphs installed and the room is sparsely furnished, as in configuration 51.8CA_F, which is also included in  Figure  7  and  Figure  8).  For  all  configurations  in  this  section,  a  full  covering  absorbent  ceiling  is  below for comparison. graphs below for comparison.  installed and the room is sparsely furnished, as in configuration 51.8CA_F, which is also included in  graphs below for comparison.  Acoustics 2020, 3 FOR PEER REVIEW   12  Acoustics 2020, 2 516 Figure 11. Room acoustic parameters, (a) sound strength (G), (b) speech clarity (C ) and (c) reverberation time (T ). Blue: base configuration with only ceiling treatment and furniture (51.8CA_F) Figure  11.  Room  acoustic  parameters,  (a)  sound  strength  (G),  (b)  speech  clarity  (C50)  and  (c)  to be compared with configurations having acoustic treatment on the walls. Red: porous absorbers reverberation  time  (T20).  Blue:  base  configuration  with  only  ceiling  treatment  and  furniture  (51.8CA_F_8.6WA). Green: vertically directed di users (51.8CA_F_8.6VWD). Violet: horizontally (51.8CA_F) to be compared with configurations having acoustic treatment on the walls. Red: porous  directed di users (51.8CA_F_8.6HWD). The di erent types of acoustic wall treatment cover the same absorbers  (51.8CA_F_8.6WA).  Green:  vertically  directed  diffusers  (51.8CA_F_8.6VWD).  Violet:  area in all three cases. The strongest impact on G is achieved with wall absorbers. Di users also had a horizontally directed diffusers (51.8CA_F_8.6HWD). The different types of acoustic wall treatment  minor e ect on this parameter, with similar values obtained independent of orientation. For C and cover the same area in all three cases. The strongest impact on G is achieved with wall absorbers.  T , the orientation of di users is critical, with a greater e ect achieved by vertically directed di users. Diffusers  also had  a  minor effect  on  this  parameter,  with similar values  obtained independent  of  The configuration with wall absorbers results in lower sound strength values in a frequency range of orientation. For C50 and T20, the orientation of diffusers is critical, with a greater effect achieved by  250 Hz to 4000 Hz, both in comparison with di user configurations, as well as with configurations with vertically directed diffusers.  no wall treatment; a decrease is seen for the entire frequency range in this comparison. The di erences are small, The configuration but a clear tr with end iswa ap ll par absorb ent ers (see results the graph  in loin wer Figur  soun e d 11 st ).rength Up to values 0.8 dB, inlower  a freq values uency  ar range e obtained   of  250 for Hzthe   to configur 4000  Hz, ation   both with   in  comparison absorbing wall   with tr eatment diffuser and conf the igur diat io using ns,  as wall   well tr eatment as  with  configurations  with  no  wall  treatment;  a  decrease  is  seen  for  the  entire  frequency  range  in  this  within the frequency range of 250–4000 Hz. Thus, G is still lower for configurations with di users, compar comparison ed to. The no wall  differenc treatment es are(51.8CA_F).  small, but a Further clear trend , a simil  is appa ar Grent is obtained  (see the for graph configurations  in Figure 11). with  Up  to 0.8 dB, lower values are obtained for the configuration with absorbing wall treatment and the  di user treatment (51.8CA_F_8.6VWD) and (51.8CA_F_8.6HWD), i.e., the values for this parameter diffusing  wall  treatment  within  the  frequency  range  of  250–4000  Hz.  Thus,  G  is  still  lower  for  are independent of the direction of di users. Note also from this graph, that the lower values in G for configurations  with  diffusers,  compared  to  no  wall  treatment  (51.8CA_F).  Further,  a  similar  G  is  configurations with di users is at a frequency of 125 Hz. This decrease is not correlated to scattering, obtained for configurations with diffuser treatment (51.8CA_F_8.6VWD) and (51.8CA_F_8.6HWD),  but it is due to the resonance absorption for this frequency included in the design of the di users used i.e., the values for this parameter are independent of the direction of diffusers. Note also from this  in the study. This is shown further in Section 3.4. graph, that the lower values in G for configurations with diffusers is at a frequency of 125 Hz. This  Speech clarity increases for all configurations with any type of wall treatment. The largest decrease is not correlated to scattering, but it is due to the resonance absorption for this frequency  increase is seen for configuration with wall absorbers (51.8CA_8.6WA) at a frequency of 500–4000 Hz. included in the design of the diffusers used in the study. This is shown further in Section 3.4.  In comparison with di users, the change is largest at 4000 Hz, with a 1.7 dB and 2.8 dB di erence for Speech  clarity  increases  for  all  configurations  with  any  type  of  wall  treatment.  The  largest  vertically oriented and horizontally oriented, respectively. increase is seen for configuration with wall absorbers (51.8CA_8.6WA) at a frequency of 500–4000  The two configurations with di users have similar C values in octave bands 125 Hz to 500 Hz, Hz. In comparison with diffusers, the change is largest at 4000 Hz, with a 1.7 dB and 2.8 dB difference  i.e., in the range where these di users are not designed to be e ective. However, at 1000 Hz to 4000 Hz, for vertically oriented and horizontally oriented, respectively.  clearly higher C values are obtained for the vertically oriented di users—about 0.8–1.0 dB higher, The two configurations with diffusers have similar C50 values in octave bands 125 Hz to 500 Hz,  compared to horizontally oriented di users. This is a frequency range where the di users are e ective, i.e., in the range where these diffusers are not designed to be effective. However, at 1000 Hz to 4000  but the vertically oriented di users, to a greater degree, distract the lateral sound field and redirect the Hz,  clearly  higher  C50  values  are  obtained  for  the  vertically  oriented  diffusers—about  0.8–1.0  dB  sound to the absorbent ceiling. The same behavior between the di erent configurations is seen for higher, compared to horizontally oriented diffusers. This is a frequency range where the diffusers are  reverberation time. In addition to this, for the higher frequencies, similar results are achieved in T effective, but the vertically oriented diffusers, to a greater degree, distract the lateral sound field and  for configuration with vertical di users and wall absorbers. A change in the behavior is obtained for redirect the sound to the absorbent ceiling. The same behavior between the different configurations  frequency 125 Hz for both C and T in configurations with di users. As for sound strength, this is 50 20 is  seen  for  reverberation  time.  In  addition  to  this,  for  the  higher  frequencies,  similar  results  are  due to resonance absorption in the di users. achieved in T20 for configuration with vertical diffusers and wall absorbers. A change in the behavior  is obtained for frequency 125 Hz for both C50 and T20 in configurations with diffusers. As for sound  strength, this is due to resonance absorption in the diffusers.  Acoustics 2020, 2 517 Acoustics 2020, 3 FOR PEER REVIEW   13  Acoustics 2020, 3 FOR PEER REVIEW   13  3.3. Ceiling Di users 3.3. Ceiling Diffusers  3.3. Ceiling Diffusers  The following section presents the e ect of di users installed in the ceiling. Six of the absorbent The following section presents the effect of diffusers installed in the ceiling. Six of the absorbent  The following section presents the effect of diffusers installed in the ceiling. Six of the absorbent  panels in the front of the room, typical speaker position, were replaced with di users, corresponding panels in the front of the room, typical speaker position, were replaced with diffusers, corresponding  panels in 2  the front of the room, typical speaker position, were 2  replaced with diffusers, corresponding  to 4.3 m , see Figure 12. The walls are covered with 8.6 m of absorbers and the room is sparsely 2 2 to 4.3 m , see Figure 12. The walls are covered with 8.6 m  of absorbers and the room is sparsely  2 2 to 4.3 m , see Figure 12. The walls are covered with 8.6 m  of absorbers and the room is sparsely  furnished (47.5CA_4.3CD_F_8.6WA). furnished (47.5CA_4.3CD_F_8.6WA).  furnished (47.5CA_4.3CD_F_8.6WA).  Figure 12. Location of ceiling diffusers above assumed speaker position. R2 corresponds to position  Figure 12. Location of ceiling diffusers above assumed speaker position. R2 corresponds to position  Figure 12. Location of ceiling di users above assumed speaker position. R2 corresponds to position for listener close to the speaker, R5 corresponds to listener in the rear area of the room.  for listener close to the speaker, R5 corresponds to listener in the rear area of the room.  for listener close to the speaker, R5 corresponds to listener in the rear area of the room. The results for the average values over the twelve measurements show a general decrease in G  The The r esults results for fothe r the average  averag ve alues  values over over the th twelve e twelmeasur ve measurements ements show show a general  a gener decr al ease decre inase G and  in G  and T20, and an increase of C50. The difference is small but a clear trend is obtained, see Figure 13.  Tand , and  T20,an and incr an ease  incre of a Cse .of The  C50di . The erence  difference is small isbut  smaall clear  but tra end cleais r trend obtained,  is obtained, see Figur see e 13 Figure . Ceiling  13.  20 50 Ceiling  diffuser  configurations  were  also  tested  with  no  wall  treatment  or  wall  diffusers,  with  diCeil user ingconfigurations   diffuser  config wer ureaalso tions tested   werewith   alsono  tested wall trwi eatment th  no  or wall wall   trea di tment users,  or with   wall equivalent   diffusertr s,ends   with  equivalent trends being obtained (for the results, see Appendix A).  being equivalent obtained  trends (for being the results,  obtained see Appendix (for the results, A).  see Appendix A).  Figure 13. Global figures, averaged over all source and receiver positions. In (a) sound strength (G), Figure 13. Global figures, averaged over all source and receiver positions. In (a) sound strength (G),  Figure 13. Global figures, averaged over all source and receiver positions. In (a) sound strength (G),  (b) speech clarity (C ) and (c) reverberation time (T ). Blue: room with absorbent ceiling, furniture 50 20 (b) speech clarity (C50) and (c) reverberation time (T20). Blue: room with absorbent ceiling, furniture  (b) speech clarity (C50) and (c) reverberation time (T20). Blue: room with absorbent ceiling, furniture  and wall absorbers (51.8CA_F_8.6WA). Red: room with partly absorbent ceiling, partly ceiling di users, and  wall  absorbers  (51.8CA_F_8.6WA).  Red:  room  with  partly  absorbent  ceiling,  partly  ceiling  and  wall  absorbers  (51.8CA_F_8.6WA).  Red:  room  with  partly  absorbent  ceiling,  partly  ceiling  furniture and wall absorbers (47.5CA_4.3CD_F_8.6WA). Configuration with ceiling di users gives diffusers,  furniture  and  wall  absorbers  (47.5CA_4.3CD_F_8.6WA).  Configuration  with  ceiling  diffusers,  furniture  and  wall  absorbers  (47.5CA_4.3CD_F_8.6WA).  Configuration  with  ceiling  lower value in G, with a clear trend apparent even if the di erence is small. In terms of C ceiling diffusers gives lower value in G, with a clear trend apparent even if the difference is small. In terms  diffusers gives lower value in G, with a clear trend apparent even if the difference is small. In terms  di users give an increase from frequency 500 Hz and upwards, with the greatest di erence being of  C50  ceiling  diffusers  give  an  increase  from  frequency  500  Hz  and  upwards,  with  the  greatest  of  C50  ceiling  diffusers  give  an  increase  from  frequency  500  Hz  and  upwards,  with  the  greatest  0.8 dB, at 1000 Hz. The reverberation time decreased in configuration with ceiling di users over the difference being 0.8 dB, at 1000 Hz. The reverberation time decreased in configuration with ceiling  difference being 0.8 dB, at 1000 Hz. The reverberation time decreased in configuration with ceiling  entire frequency range, with the change being small but the trend clear. diffusers over the entire frequency range, with the change being small but the trend clear.  diffusers over the entire frequency range, with the change being small but the trend clear.  The e ect of ceiling di users was further evaluated for di erent positions in the room by comparing The  effect  of  ceiling  diffusers  was  further  evaluated  for  different  positions  in  the  room  by  The  effect  of  ceiling  diffusers  was  further  evaluated  for  different  positions  in  the  room  by  the room acoustic parameters for receiver positions R2 and R5, source position S2. Positions are comparing  the  room  acoustic  parameters  for  receiver  positions  R2  and  R5,  source  position  S2.  comparing  the  room  acoustic  parameters  for  receiver  positions  R2  and  R5,  source  position  S2.  described in Figure 3 and further visualized in Figure 12. Positions are described in Figure 3 and further visualized in Figure 12.  Positions are described in Figure 3 and further visualized in Figure 12.  Sound strength decreases with distance from the source, but a comparison to 51.8CA_F_8.6WA  Sound strength decreases with distance from the source, but a comparison to 51.8CA_F_8.6WA  show higher values for configuration with ceiling diffusers 47.5CA_4.3CD_F_8.6WA. In addition to  show higher values for configuration with ceiling diffusers 47.5CA_4.3CD_F_8.6WA. In addition to  Acoustics 2020, 2 518 Acoustics 2020, 3 FOR PEER REVIEW   14  Sound strength decreases with distance from the source, but a comparison to 51.8CA_F_8.6WA Acoustics 2020, 3 FOR PEER REVIEW   14  the increased energy level, a significant increase is obtained for C50. In R5, i.e., in the back of the room,  show higher values for configuration with ceiling di users 47.5CA_4.3CD_F_8.6WA. In addition to the a difference can be seen for the entire frequency range, with strongest effect at 1000–4000 Hz, an  the increased energy level, a significant increase is obtained for C50. In R5, i.e., in the back of the room,  increased energy level, a significant increase is obtained for C . In R5, i.e., in the back of the room, a increase of 1.5–3.2 dB. The reverberation time is also affected, mainly in the higher frequency range.  a difference can be seen for the entire frequency range, with strongest effect at 1000–4000 Hz, an  di erence can be seen for the entire frequency range, with strongest e ect at 1000–4000 Hz, an increase Results are shown in Figure 14 and Figure 15.  increase of 1.5–3.2 dB. The reverberation time is also affected, mainly in the higher frequency range.  of 1.5–3.2 dB. The reverberation time is also a ected, mainly in the higher frequency range. Results are Results are shown in Figure 14 and Figure 15.  shown in Figures 14 and 15. Figure 14. In (a) sound strength (G), (b) speech clarity (C50) and (c) reverberation time (T20) in position  R2.  Blue:  without  ceiling  diffusers  (51.8CA_F_8.6WA).  Red:  with  ceiling  diffusers  Figure 14. In (a) sound strength (G), (b) speech clarity (C ) and (c) reverberation time (T ) in position R2. 50 20 Figure 14. In (a) sound strength (G), (b) speech clarity (C50) and (c) reverberation time (T20) in position  (47.5CA_4.3CD_F_8.6WA). Small increases in G and C50 are achieved in configurations with diffusers  Blue: without ceiling di users (51.8CA_F_8.6WA). Red: with ceiling di users (47.5CA_4.3CD_F_8.6WA). R2.  Blue:  without  ceiling  diffusers  (51.8CA_F_8.6WA).  Red:  with  ceiling  diffusers  in combinations with slightly lower T20.  Small increases in G and C are achieved in configurations with di users in combinations with slightly (47.5CA_4.3CD_F_8.6WA). Small increases in G and C50 are achieved in configurations with diffusers  lower T . in combinations with slightly lower T20.  Figure 15. In (a) sound strength (G), (b) speech clarity (C ) and (c) reverberation time (T ) in position 50 20 Figure 15. In (a) sound strength (G), (b) speech clarity (C50) and (c) reverberation time (T20) in position  R5, back of the room. Blue: without ceiling di users (51.8CA_F_8.6WA). Red: with ceiling di users R5, back of the room. Blue: without ceiling diffusers (51.8CA_F_8.6WA). Red: with ceiling diffusers  Figure 15. In (a) sound strength (G), (b) speech clarity (C50) and (c) reverberation time (T20) in position  (47.5CA_4.3CD_F_8.6WA). A small increase is seen for G. A significant increase in C is achieved with (47.5CA_4.3CD_F_8.6WA). A small increase is seen for G. A significant increase in C50 is achieved  R5, back of the room. Blue: without ceiling diffusers (51.8CA_F_8.6WA). Red: with ceiling diffusers  slightly lower T . The strongest improvements are seen in position R5 and in the frequency range of with slightly lower T20. The strongest improvements are seen in position R5 and in the frequency  (47.5CA_4.3CD_F_8.6WA). A small increase is seen for G. A significant increase in C50 is achieved  1000–4000 Hz. range of 1000—4000 Hz.  with slightly lower T20. The strongest improvements are seen in position R5 and in the frequency  3.4. Combining Di users with Helmholtz Absorption range of 1000—4000 Hz.  3.4. Combining Diffusers with Helmholtz Absorption  One way to obtain good absorption in a certain frequency is to use resonance absorbers. 3.4. Combining One way  Diffusers to obtain wi good th Helmh  absorp oltzt iAbsorption on in a certain    frequency is to use resonance absorbers. The  The di users used in this study were designed to operate as Helmholtz resonator at frequencies diffusers used in this study were designed to operate as Helmholtz resonator at frequencies 125–250  One way to obtain good absorption in a certain frequency is to use resonance absorbers. The  125–250 Hz. The result of its e ect in configuration 51.8CA_F_8.6VWD is compared with configuration Hz. The result of its effect in configuration 51.8CA_F_8.6VWD is compared with configuration with  with diffusers a por use ousd low in thi frsequency  study were absorber  desig,ned configuration  to operate as 51.8CA_25.0LFMA,  Helmholtz resonadescribed tor at frequenc in Section ies 125–2 2.4.1 50.  a  porous  low  frequency  absorber,  configuration  51.8CA_25.0LFMA,  described  in  Section  2.4.1.  Hz. The result of its effect in configuration 51.8CA_F_8.6VWD is compared with configuration with  Evaluation of reverberation time shows a clear e ect for both configurations at low frequencies. Evaluation of reverberation time shows a clear effect for both configurations at low frequencies. It  It a could porous be  lo noted w  fre that quency 8.6 m  ab diso rb users er,  co wer nfig e used uration while   51the .8CA ar_2 ea5. of0L por FMA, ous  low described frequency   in  Section absorber  2.4.1. was  could be noted that 8.6 m  diffusers were used while the area of porous low frequency absorber was  Evaluation of reverberation time shows a clear effect for both configurations at low frequencies. It  25 m . Additionally, the diffusers affect the higher frequency range to a greater extent. The results are  could be noted that 8.6 m  diffusers were used while the area of porous low frequency absorber was  presented in Figure 16.  25 m . Additionally, the diffusers affect the higher frequency range to a greater extent. The results are  presented in Figure 16.  Acoustics 2020, 2 519 25 m . Additionally, the di users a ect the higher frequency range to a greater extent. The results are Acoustics 2020, 3 FOR PEER REVIEW   15  presented in Figure 16. Figure 16. Reverberation time (T ), in blue only absorbent ceiling (51.8CA), in green vertical wall Figure 16. Reverberation time (T20), in blue only absorbent ceiling (51.8CA), in green vertical wall  di users (51.8CA_8.6VWD) and in red low frequency porous absorber (51.8CA_25.0LFMA). The two diffusers (51.8CA_8.6VWD) and in red low frequency porous absorber (51.8CA_25.0LFMA). The two  latter configurations a ect the lower frequencies significantly; thus, the areas used for the di erent latter configurations affect the lower frequencies significantly; thus, the areas used for the different  low frequency treatments are di erent. In addition, the di users a ect the higher frequency range to a low frequency treatments are different. In addition, the diffusers affect the higher frequency range to  greater extent. a greater extent.  4. Discussion 4. Discussion  Installing the fully covering absorbent ceiling in the empty room, as a first step, a ected all the room acoustic parameters. This treatment can be seen as a good baseline for a classroom, since it Installing the fully covering absorbent ceiling in the empty room, as a first step, affected all the  significantly decreases the sound strength and reverberation time, and increases the speech clarity. room acoustic parameters. This treatment can be seen as a good baseline for a classroom, since it  The addition of furniture also a ected the results, mainly due to increased scattering, as only a small significantly decreases the sound strength and reverberation time, and increases the speech clarity.  amount of absorption is involved. The furniture a ects the parameters particularly at frequencies of The addition of furniture also affected the results, mainly due to increased scattering, as only a small  500–1000 Hz. This configuration, an absorbent ceiling and a sparsely furnished room, could be seen as amount of absorption is involved. The furniture affects the parameters particularly at frequencies of  a normal classroom situation. It is important to note the di erence in result between measurement 500–1000 Hz. This configuration, an absorbent ceiling and a sparsely furnished room, could be seen  and calculated T by using di use sound field theory for this configuration. The much lower values achieved in calculation demonstrate that the lateral sound field must be considered in acoustic models as a normal classroom situation. It is important to note the difference in result between measurement  dealing with ordinary, furnished rooms. and calculated T20 by using diffuse sound field theory for this configuration. The much lower values  The additional acoustic treatment in di erent configurations using absorbers and di users achieved  in  calculation  demonstrate  that  the  lateral  sound  field  must  be  considered  in  acoustic  contributed important e ects for fine-tuning the acoustics. The general finding was that higher models dealing with ordinary, furnished rooms.  sound energy levels were obtained for configurations with di users, and lower energy levels for The  additional  acoustic  treatment  in  different  configurations  using  absorbers  and  diffusers  configurations with absorbers. Both types of treatment a ected speech clarity and reverberation time. contributed important effects for fine‐tuning the acoustics. The general finding was that higher sound  With the di users, the energy is conserved, compared to the absorbers where the energy is energy  levels  were  obtained  for  configurations  with  diffusers,  and  lower  energy  levels  for  reduced, explaining why both horizontally oriented and vertically oriented di users have similar configurations with absorbers. Both types of treatment affected speech clarity and reverberation time.  values in terms of sound strength. A reason for the e ect on speech clarity and reverberation time is With  the  diffusers,  the  energy  is  conserved,  compared  to  the  absorbers  where  the  energy  is  the scattering of the di users. Significant di erences are found between the vertically and horizontally reduced directing , expdi lain users, ing wh wher y bo e the th vertical horizont a ally ected oriented C and Tand to vert a gric eater ally  extent. oriented The di vertically ffusers ha oriented ve similar  50 20 di users reduced the sound waves in the horizontal plane and directed the sound into the ceiling, values in terms of sound strength. A reason for the effect on speech clarity and reverberation time is  showing the importance of directional scattering in rooms with ceiling treatment, which correlates the  scattering  of  the  diffusers.  Significant  differences  are  found  between  the  vertically  and  well to the di usion characteristics of the di using elements. horizontally  directing  diffusers,  where  the  vertical  affected  C50  and  T20  to  a  greater  extent.  The  In the experiments using di users in the ceiling, important improvements for receivers located vertically oriented diffusers reduced the sound waves in the horizontal plane and directed the sound  outside the direct sound field could be achieved. Keeping the sound energy level, a significant increase into the ceiling, showing the importance of directional scattering in rooms with ceiling treatment,  in speech clarity was found for the receiver at the back. This is an important application for use in which correlates well to the diffusion characteristics of the diffusing elements.  classrooms where a teacher gives instructions at the front of the room. An interesting finding with In the experiments using diffusers in the ceiling, important improvements for receivers located  outside  the  direct  sound  field  could  be  achieved.  Keeping  the  sound  energy  level,  a  significant  increase in speech clarity was found for the receiver at the back. This is an important application for  use in classrooms where a teacher gives instructions at the front of the room. An interesting finding  with all the ceiling diffuser configurations was that a decrease in T20 was obtained while G increased,  and this finding also applied when evaluating the results on an average basis for the twelve different  measurements.  This  raises,  again,  the  question  of  the  importance  of  considering  the  effect  of  scattering.  Acoustics 2020, 2 520 all the ceiling di user configurations was that a decrease in T was obtained while G increased, and this finding also applied when evaluating the results on an average basis for the twelve di erent measurements. This raises, again, the question of the importance of considering the e ect of scattering. It should be observed that C only gives information about the early-to-late ratio of the reflections, and does not explain anything about the sound energy. This means that high C can be achieved without guaranteeing sound energy will be suciently high for the listener, or supporting the speaker. A case with high C and low sound energy can result in: (1) Too low a sound level reaching listeners in the rear area of the classroom; (2) Greater voice e ort for the speaker. In the configurations with di users, an increase in C could retain the sound strength, i.e., the sound energy. In choosing the acoustic treatment, it is important to consider the type of room acoustic properties required for the specific room. With di users, reverberation time can be lowered and the ratio of early reflections increase with sound energy conserved. Thus, for environments where complex tasks requiring concentration are performed or in a very noisy environment, sound energy reducing treatment should be used. It is thus of importance to define the activity taking place in the room when choosing the acoustic treatment. In the discussion of acoustic design, it is important to note that the di users used in this study were designed to operate as scattering objects for the higher frequency range, which is important for speech, and for absorption at low frequencies. It is possible to design the di users to operate at the requested frequencies. It has been mentioned above that only small e ects were seen for some configurations. It should be noted that only a small part of the wall area was covered, for example, the ceiling di users covered 8% of the ceiling area. An increased area of acoustic treatment would, to a certain degree, a ect the values further. However, realistic conditions must be considered, e.g., a real classroom can have more furniture a ecting the scattering properties. Other factors that can influence the perception of the acoustics, not studied in this investigation, are background noise, the people and their activities. An observation from the results is the importance of using several room acoustic parameters to characterize a specific acoustic environment (see e.g., Figure 15). For example, T and C can be 20 50 varied with di users, but still maintain the sound strength. However, using absorbers, the sound strength can be varied as well. The outcome of these two scenarios will be di erent experienced, and needs to be considered to obtain the correct acoustic balance for low and high frequencies in ordinary public rooms. 5. Conclusions A clear trend in how di erent types of acoustic treatment a ect room acoustic parameters has been demonstrated where, as a baseline, an acoustic ceiling should be used to decrease the energy level, increase the ratio of early-to-late reflections and lower the reverberation time. However, in a sparsely furnished room, it can still be dicult to achieve a high-quality sound environment with only an absorbent ceiling. The room acoustic parameters can be fine-tuned by using di users and absorbers on the walls and/or in the ceiling. The two di erent types of treatment operate di erently and create distinctive experiences for the people in the room. With additional porous absorbers, G and T are decreased and C is increased, while di users a ect C and T , keeping G stable or increased, 20 50 50 20 depending on placement and amount. The di users used in this study where ecient in absorbing sound at the frequency of 125–250 Hz, but the frequency range within which it operates as a resonant absorber can easily be adopted. Additionally, the frequency ranges for which it operated most eciently as a scattering object can be adjusted, depending on the demands. Acoustics 2020, 2 521 This study provides information on how di erent acoustic treatments can be used to obtain di erent room acoustic qualities, and can be used to improve the sound environment in ordinary public rooms. However, the target values of the room acoustic parameters must be defined for the specific environment and activity, in order to use this information for correct the fine-tuning of acoustic environments. 6. Patents Patent pending, European Patent Oce, 20170667.8. Author Contributions: Writing—original draft preparation: E.A.; Writing—significant contribution to introduction paragraph: E.N.; Experiments—design and accomplishment: E.A. and E.N.; Experiments—analysis: E.A.; Writing—editing and review: E.A.; Structure of paper and supervision: E.N., O.J.I.K. and D.B.H. All authors read and agreed to the published version of the manuscript. Funding: This research is funded by Saint-Gobain Ecophon. Acknowledgments: The authors are grateful for the support from Ecophon providing material, laboratory equipment and laboratory facilities making it possible to perform the experiments. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations Acoustic Configurations CA Ceiling absorptive CD Ceiling di user F Furniture, the room is sparsely furnished WA Wall absorbers VWD Vertical wall di users HWD Horizontal wall di users LFMA Low frequency mineral wool absorber Appendix A Table A1. Number together with any of above written abbreviations describes the m of the specified acoustic treatment. Acoustic Parameters G Sound Strength C Speech Clarity T Reverberation time, evaluated over 20 dB decrease References 1. Astolfi, A.; Pellerey, F. Subjective and objective assessment of acoustical and overall environmental quality in secondary school classrooms. J. Acoust. Soc. Am. 2008, 123, 163–173. [CrossRef] [PubMed] 2. Shield, B.; Conetta, R.; Dockrell, J.; Connolly, D.; Cox, T.; Mydlarz, C. A survey of acoustic conditions and noise levels in secondary school classrooms in England. J. Acoust. Soc. Am. 2015, 137, 177–188. [CrossRef] [PubMed] 3. Ljung, R.; Israelsson, K.; Hygge, S. Speech intelligibility and recall of spoken material heard at di erent signal-to-noise ratios and the role played by working memory capacity. Appl. Cogn. Psychol. 2013, 27, 198–203. [CrossRef] 4. Shield, B.M.; Dockrell, J.E. The e ects of environmental and classroom noise on the academic attainments of primary school children. J. Acoust. Soc. Am. 2008, 123, 133–144. [CrossRef] 5. Åhlander, V.L.; Rydell, R.; Löfqvist, A. Speaker ’s comfort in teaching environments: Voice problems in Swedish teaching sta . J. Voice 2011, 25, 430–440. [CrossRef] Acoustics 2020, 2 522 6. Pelegrín-García, D.; Brunskog, J. Speakers’ comfort and voice level variation in classrooms: Laboratory research. J. Acoust. Soc. Am. 2012, 132, 249–260. [CrossRef] 7. Brunskog, J.; Gade, A.; Bellester, G.; Calbo, L. Increase in voice level and speaker comfort in lecture rooms. J. Acoust. Soc. Am. 2009, 125, 2072–2082. [CrossRef] 8. Pelegrin-Garcia, D.; Brunskog, J.; Lyberg-Åhlander, V.; Löfqvist, A. Measurement and prediction of voice support and room gain in school classrooms. J. Acoust. Soc. Am. 2012, 13, 194–204. [CrossRef] 9. Rasmussen, B.; Brunskog, J.; Ho meyer, D. Reverberation time in class rooms–Comparison of regulations and classification criteria in the Nordic countries. In Proceedings of the Joint Baltic-Nordic Acoustic Meeting, Odense, Denmark, 18 June 2012. 10. ISO 3382-2. Acoustics-Measurement of Room Acousitc Parameters-Part. 2: Reverberation Time in Ordinary Rooms; International Organization for Standardization: Brussels, Belgium, 2008. 11. Cremer, L.; Müller, A.; Schultz, T. Principles and applications of room acosutics. Appl. Sci. Publ. 1982, 1, 413. 12. Kuttru , H. Room Acoustics; CRC Press: Boca Raton, FL, USA, 2017; p. 174. 13. Nilsson, E. Decay processes in rooms with non-di use sound fields. Part 1: Ceiling treatment with absorbing material. J. Build. Acosutics 2004, 11, 39–60. [CrossRef] 14. Lochner, J.; Burger, F. The influence of reflections on auditorium acoustics. J. Sound Vib. 1964, 4, 426–454. [CrossRef] 15. Sato, J.; Bradley, J. Evaluation of acoustical conditions for speech communication in working elementary school classrooms. J. Acoust. Soc. Am. 2008, 4, 2064–2077. [CrossRef] [PubMed] 16. Sato, H.; Morimoto, M.; Sato, H.; Wada, M. Relationship between listening diculty and acoustical objective measures in reverberant fields. J. Acoust. Soc. Am. 2008, 4, 2087–2093. [CrossRef] [PubMed] 17. Sato, H.; Morimoto, M.; Wada, M. Relationship between listening diculty rating and objective measures in reverberant and noisy sound fields for young adults and elderly persons. J. Acoust. Soc. Am. 2012, 6, 4596–4605. [CrossRef] 18. Sato, H.; Bradley, J.; Morimoto, M. Using listening diculty ratings of conditions for speech communication in rooms. J. Acoust. Soc. Am. 2005, 3, 1157–1167. 19. Yang, W.; Bradley, J. E ects of room acoustics on the intelligibility of speech in classrooms for young children. J. Acoust. Soc. Am. 2009, 2, 922–933. [CrossRef] 20. Nijs, L.; Rychtarikova, M. Calculating the optimum reverberation time and absorption coecient for good speech intelligibility in classroom design using U50. Acta Acoust. United Acoust. 2011, 1, 93–102. [CrossRef] 21. Astolfi, A.; Parati, L.; D’Orazio, D.; Garai, M. The New Italian standard UNI 11532 on acoustics for schools. In International Congress on Acoustics; ICA: Aachen, Germany, 2019. 22. NS. 8175:2019, Lydforhold I Bygninger Lydklasser for Ulike Bygningstyper; Norwegian Standard: Utrecht, The Netherlands, 2019. 23. IEC. 60268-16: Sound System Equipment. Part. 16, ObJ.ective Rating of Speech Intelligibility by Speech Transmission Index, 2011–06; IEC: Geneva, Switzerland, 2011. 24. EN. ISO 3382-1: Acoustics-Measurement of Room Acoustic Parameters-Part. 1: Performance Spaces; Norwegian Standard: Utrecht, The Netherlands, 2009. 25. Puglisi, G.E.; Prato, A.; Sacco, T.; Astolfi, A. Influence of classroom acoustics on the reading speed: A case study on Italian second-graders. J. Acoust. Soc. Am. 2018, 144, 2. [CrossRef] 26. Bradley, J.S.; Reich, R.; Norcross, S. On the combined e ects of signal-to-noise ratio and room acoustics on speech intelligibility. J. Acoust. Soc. Am. 1999, 106, 1820–1880. [CrossRef] 27. Bradley, J.S.; Sato, H.; Picard, M. On the importance of early reflections for speech in rooms. J. Acoust. Soc. Am. 2003, 6, 3233–3244. [CrossRef] 28. Barron, M.; Lee, L.-J. Energy relations in concert auditoriums. J. Acoust. Soc. Am. 1988, 2, 618–628. [CrossRef] 29. Barron, M. Theory and measurement of early, late and total sound levels in rooms. J. Acoust. Soc. Am. 2015, 6, 3087–3098. [CrossRef] 30. Nilsson, E. Decay Processes in Room with Non-Di use Sound Fields. Part II: E ect of Irregularities. Build. Acoust. 2004, 11, 133–143. [CrossRef] 31. Cox, T.; D’Antonio, P. Acoustic Absorbers and Di users, 3rd ed.; CRC Press: Boca Raton, FL, USA, 2017. 32. Cucharero, J.; Hänninen, T.; Lokki, T. Influence of sound-absorbing material placement on room acoustical parameters. In Acoustics; Multidisciplinary Digital Publishing Institute: Basel, Switzerland, 2019. Acoustics 2020, 2 523 33. Berardi, U.; Iannace, G.; Trematerra, A. Acoustic treatments aiming to achieve the italian minimum environmental criteria (cam) standards in large reverberant classrooms. Can. Acoust. 2019, 47, 1. 34. Choi, Y.-J. An Optimum Combination of Absorptive and Di using Treatments for Classroom Acosutic Design. Build. Acoust. 2014, 2, 175–180. [CrossRef] 35. Choi, Y.-J. The application of di users for classroom acoustical design. Noise Vib. Worldw. 2014, 45, 5. 36. Nilsson, E.; Hammer, P. Subjective evaluation of speech intelligibility for normal-hearing persons and for persons with simulated minimal degrees of hearing-loss. In INTER-NOISE and NOISE-CON Congress and Conference Proceedings; Institute of Noise Control Engineering: Reston, VA, USA, 1995. 37. Knüttel, T.; Vorländer, M.; Witew, I.B. Influence of omnidirectional loudspeaker directivity on measured room impulse responses. J. Acoust. Soc. Am. 2013, 5, 3654–3662. [CrossRef] 38. Vigran, T. Building Acoustics; Taylor and Francis: London, UK, 2008; p. 452. 39. SS-EN ISO. 11654:1997, Byggakustik-LJ.udabsorbenter-Värdering av Mätresultat Och Klassindelning; ISO: Geneva, Switzerland, 1997. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Acoustics Multidisciplinary Digital Publishing Institute

The Effect on Room Acoustical Parameters Using a Combination of Absorbers and Diffusers—An Experimental Study in a Classroom

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acoustics Article The E ect on Room Acoustical Parameters Using a Combination of Absorbers and Di users—An Experimental Study in a Classroom 1 , 2 1 2 Emma Arvidsson * , Erling Nilsson , Delphine Bard Hagberg and Ola J. I. Karlsson Engineering Acoustics, Lund University, John Ericssons väg 1, 221 00 Lund, Sweden; Delphine.Bard@construction.lth.se Saint-Gobain Ecophon AB, Yttervägen 1, 265 75 Hyllinge, Sweden; erling.nilsson@ecophon.se (E.N.); ola.karlsson@ecophon.se (O.J.I.K.) * Correspondence: emma.arvidsson@construction.lth.se Received: 9 June 2020; Accepted: 2 July 2020; Published: 4 July 2020 Abstract: Several room acoustic parameters have to be considered in ordinary public rooms, such as oces and classrooms, in order to present the actual conditions, thus increasing demands on the acoustic treatment. The most common acoustical treatment in ordinary rooms is a suspended absorbent ceiling. Due to the non-uniform distribution of the absorbent material, the classical di use field assumption is not fulfilled in such cases. Further, the sound scattering e ect of non-absorbing objects such as furniture are considerable in these types of rooms. Even the directional characteristic of the sound scattering objects are of importance. The sound decay curve in rooms with absorbent ceilings often demonstrate a double slope. Thus, it is not possible to use reverberation time as room parameter as a representative standalone acoustic measure. An evaluation that captures the true room acoustical conditions therefore needs supplementary parameters. The aim of this experimental study is to show how various acoustical treatments a ect reverberation time T , speech clarity C 20 50 and sound strength G. The experiment was performed in a mock-up of a classroom. The results demonstrated how absorbers, di users and scattering objects influence room acoustical parameters. It is shown that to some extent the parameters can be adjusted individually by using di erent treatments or combination of treatments. This allows for the fine-tuning of the acoustical conditions, in order to fulfill the requirements for achieving a high-quality sound environment. Keywords: room acoustics; sound di usion; sound absorption; sound scattering; sound strength; speech clarity; reverberation time 1. Introduction In ordinary public rooms, the typical acoustic treatment is a suspended absorbent ceiling. Examples of ordinary public rooms are classrooms, oces, health care premises and restaurants. Many people spend their working days in those spaces performing a variety of di erent activities. The acoustical conditions are crucial for people’s wellbeing and while supporting their activities. 1.1. Room Acoustic Parameters in Classrooms The importance of good acoustical conditions in schools, with classrooms that support speech communication, as well as concentrated work, is well documented. Several surveys in school environments have emphasized the detrimental e ects of insucient acoustic treatment in classrooms [1,2]. The e ect on cognitive functions, such as working memory, have been investigated [3], as well as on academic attainment [4]. Acoustical treatment in classrooms should not only secure good Acoustics 2020, 2, 505–523; doi:10.3390/acoustics2030027 www.mdpi.com/journal/acoustics Acoustics 2020, 2 506 listening conditions; it has been reported that teachers su er from voice disorders to a greater degree than the rest of the population [5]. Objective measures for voice support in medium-sized classrooms have therefore been developed [6–8]. The most common way to specify room acoustical target values in standards and regulations is to use reverberation time T [9]. The reverberation time is often evaluated as T , i.e., evaluating the 60 20 range5 to25 dB of the decay curve. Validation is defined in ISO 3382-2 [10]. Due to the procedure for evaluation of T , early reflections are ignored. As stated in textbooks on acoustics [11,12], it is known that two rooms with the same reverberation time can still be perceived as di erent, and that the reverberation is not solely enough to characterize room acoustical conditions. The fact that early reflections are ignored in evaluating T is a plausible explanation for the occasionally bad correlation between the perceived condition. Furthermore, in rooms with ceiling treatment, the decay curve is often double-sloped, with a steep slope at the start of the decay and a less steep slope towards the end of the decay [13]. Thus, there is an ambiguity in the evaluation of the reverberation time, due to the non-linear behavior of the decay curve. Consequently, complementary room acoustic parameters are needed, in order to capture the subjective experience of the acoustical conditions. Lochner and Burger [14] emphasized the importance of early reflections for the subjective impression of an auditorium, stating that it is the sound field and pattern of reflections that will a ect how the sound environment is experienced, rather than one single parameter. The early reflections will contribute to the direct sound and thereby to the clarity of speech. The importance of parameters including early reflections, such as speech clarity C , have also been investigated in several studies [15–20]. Another parameter for speech intelligibility is the Speech Transmission Index (STI). The parameters C and STI have been introduced in some national standards [21,22]. STI is defined in IEC60268-16 [23] and C in ISO 3382-1 [24]. C is an energy ratio for early-to-late 50 50 arriving energy expressed in dB. The time limit between early and late energy is set at 50 ms for speech. Another parameter describing the relation between early and late reflections is definition, D which is 50, expressed as a percentage. These two parameters, C and D , are exactly related. In this study C 50 50 50 has been investigated. In a recent study [25], the reading speed for Italian second graders was investigated. The study indicates a relation between reading speed and C . No correlation to reverberation time was identified. Bradley et al. [26] investigated speech intelligibility in classrooms, examining the relation between signal-to-noise ratio and room acoustic parameters. The results from [26] show that the e ect on signal-to-noise ratio is very important for speech intelligibility, and useful-to-detrimental ratios are proposed and recommended, instead of only focusing on the reverberation time. Further, they concluded that an increase in early reflections could improve signal-to-noise ratio by up to 9 dB [27]. The non-linear decay curve in a room with absorbent ceiling treatment also implies that there is a di erence in the character of the sound field at steady-state and during the latter part of the decay. It is therefore also of interest to use the parameter sound strength G defined in ISO 3382-1. This parameter is measured during steady state, and relates to how sound reflections in a room contribute to the sound pressure level. Sound strength has mainly been used for concert halls and other performance spaces [28,29], but also for the evaluation of acoustical conditions in classrooms [15]. 1.2. Room Acoustic Treatment in Classrooms The most common acoustic treatment in ordinary public rooms is a sound absorbent ceiling and traditionally, as mentioned above, the required target values are defined by the reverberation time. When most of the absorption in ordinary rooms is predominately located at the ceiling, the scattering properties of furniture and other interior equipment will a ect the acoustical conditions [30]. In fact, even the directional characteristics of sound scattering objects in a room with a suspended absorbent ceiling will be significant. If the sound scattering objects redirect the energy up onto the absorbent ceiling or in a direction towards other sound reflecting surfaces, such as walls, the outcome will be Acoustics 2020, 2 507 di erent. This circumstance also presents the possibility of using sound scattering objects in order to fine tune acoustical conditions. Di users have long been used and applied in concert halls and studios [31]. The purpose of di users is to avoid flutter echoes and to decrease the grazing sound field, but this type of treatment can also be used to direct the sound in preferable directions [32]. Absorbers reduce echoes but also decrease the sound energy levels, which can be negative in rooms such as classrooms. Acoustics 2020, 3 FOR PEER REVIEW   3  The influence of the location of sound absorbing materials on room acoustical parameters has been studied by Cucharero et al. [32]. The e ect on reverberation time, speech clarity and STI of studios [312]. The purpose of diffusers is to avoid flutter echoes and to decrease the grazing sound  sound absorbing material, and its placements in educational rooms, was calculated in a study by field, but this type of treatment can also be used to direct the sound in preferable directions [32].  Berardi et al. [33]. Choi has studied the combination of di users and absorbers on a 1/10 scale, testing Absorbers reduce echoes but also decrease the sound energy levels, which can be negative in rooms  such as classrooms.  di erent placements of absorbers and di users [34,35]. Evaluating the room acoustic parameters of The influence of the location of sound absorbing materials on room acoustical parameters has  reverberation time, speech clarity and sound strength showed that a combination of those two di erent been studied by Cucharero et al. [32]. The effect on reverberation time, speech clarity and STI of sound  types of acoustic treatment was the most preferable when considering several acoustic parameters. absorbing material, and its placements in educational rooms, was calculated in a study by Berardi et  Another aspect of classroom acoustics is the absorption of low frequencies. Listening tests show al. [33]. Choi has studied the combination of diffusers and absorbers on a 1/10 scale, testing different  placements  of  absorbers  and  diffusers  [34,35].  Evaluating  the  room  acoustic  parameters  of  preferences for configurations with good low frequency absorption, especially in cases where a high reverberation  time,  speech  clarity  and  sound  strength  showed  that  a  combination  of  those  two  ratio of low frequency sound is emitted [36]. different  types  of  acoustic  treatment  was  the  most  preferable  when  considering  several  acoustic  parameters.  1.3. Study Objective and Principal Conclusion Another aspect of classroom acoustics is the absorption of low frequencies. Listening tests show  preferences for configurations with good low frequency absorption, especially in cases where a high  The findings in the references cited above imply that the acoustic treatment in a room needs ratio of low frequency sound is emitted [36].  to deal with di erent acoustics parameters in order to achieve good acoustic quality, both for the speaker and1.3. listener  Study Objectiv . The objective e and Principal of Conclusi this study on  is to investigate the e ect of di erent types of acoustic treatment on several room acoustic parameters. The investigation was made as a series of experiments The findings in the references cited above imply that the acoustic treatment in a room needs to  deal  with  different  acoustics  parameters  in  order  to  achieve  good  acoustic  quality,  both  for  the  in a mock-up of a classroom. Configurations with porous absorbers and di users both in the ceilings speaker and listener. The objective of this study is to investigate the effect of different types of acoustic  and on the walls have been evaluated. Combinations of resonant absorbers and di users were also treatment  on  several  room  acoustic  parameters.  The  investigation  was  made  as  a  series  of  tested, in order to further investigate the possibility of improving classroom acoustics. The e ect on experiments in a mock‐up of a classroom. Configurations with porous absorbers and diffusers both  the room acoustic parameters T , C and G is evaluated. in  the  ceilings  and  on  the 20  walls 50  have  been  evaluated.  Combinations  of  resonant  absorbers  and  diffusers  were  also  tested,  in  order  to  further  investigate  the  possibility  of  improving  classroom  2. Materials acou and stics. Methods  The effect on the room acoustic parameters T20, C50 and G is evaluated.  2. Materials and Methods  2.1. Room Mock-Up 2.1. Room Mock‐Up  The experiments were conducted in a mock-up of a classroom with dimensions 7.32 m 7.57 m 3.5 m. The ceiling The covers experime 7.2 ntsm  were  7.2 cond mucted and in was  a mo installed ck‐up of a at classroom height wi 2.70 th dim m.eDimensions nsions 7.32 m × and 7.57 m coor   dinate ×  3.5  m.  The  ceiling  covers  7.2  m  ×  7.2  m  and  was  installed  at  height  2.70  m.  Dimensions  and  system are shown in Figure 1. The room has a concrete sot, linoleum floor and gypsum walls. coordinate system are shown in Figure 1. The room has a concrete soffit, linoleum floor and gypsum  One of the walls represents a facade with three windows included. There are doors on the other walls, walls. One of the walls represents a facade with three windows included. There are doors on the other  see Figure 2. walls, see Figure 2.  Figure 1. Dimensions of room used in the experiments. Coordinate system where x is the width, y is Figure 1. Dimensions of room used in the experiments. Coordinate system where x is the width, y is  the length and z is the height of the room. the length and z is the height of the room.  Acoustics 2020, 2 508 Acoustics 2020, 3 FOR PEER REVIEW   4  Figure 2. The classroom with furniture, (a) from the back, (b) from the corner, (c) from the front, Figure 2. The classroom with furniture, (a) from the back, (b) from the corner, (c) from the front, (d)  (dfrom ) from  thethe  upper upper  corner, corner  inclu , including ding coordinates coordinates  for the for room. the  room. The room of 55 m  was furnished with 11 tables and 18 slightly upholstered chairs, but no other  The room of 55 m was furnished with 11 tables and 18 slightly upholstered chairs, but no other furniture. The room was equipped with a whiteboard, a flip board screen and luminaires on the walls  furniture. The room was equipped with a whiteboard, a flip board screen and luminaires on the walls (see Figure 2).  (see Figure 2). 2.2. Room Acoustic Parameters and Measurements  2.2. Room Acoustic Parameters and Measurements The  room acoustic  parameters  evaluated  are sound  strength (G)  Equation  (1),  speech  clarity  The room acoustic parameters evaluated are sound strength (G) Equation (1), speech clarity (C50), Equation (2) and reverberation time (T20). Measurements were performed using the DIRAC  (C ), Equation (2) and reverberation time (T ). Measurements were performed using the DIRAC 50 20 system (DIRAC type 7841, v.6.0). G was measured using a constant sound power source placed on  system (DIRAC type 7841, v.6.0). G was measured using a constant sound power source placed on the the floor. An exponential sweep signal was used as excitation for evaluation of C50 and T20. In the  floor. An exponential sweep signal was used as excitation for evaluation of C and T . In the latter, latter, an omnidirectional loudspeaker with dodecahedron geometry was used. The 50 center20 of  the  an omnidirectional loudspeaker with dodecahedron geometry was used. The center of the loudspeaker loudspeaker was at 1.55 m from the floor. An omnidirectional microphone was used as a receiver at  1.20 m from the floor. Two source positions and six receiver positions have been used; for positions  was at 1.55 m from the floor. An omnidirectional microphone was used as a receiver at 1.20 m from the see Figure 3.  floor. Two source positions and six receiver positions have been used; for positions see Figure 3. Sound strength G is defined as  Sound strength G is defined as ( ) h t dt ℎ 𝑡 𝑑𝑡 G = (1) dir 𝐺 2 (1)  h (t)dt 10m 0ms ℎ 𝑡 𝑑𝑡 Speech clarity C is defined as 50ms Speech clarity C50 is defined as  h (t)dt C = R (2) h (t)dt 50ms where h(t) is the impulse response; h is the impulse response at 10 m in a free field. 10m Acoustics 2020, 3 FOR PEER REVIEW   5  ℎ 𝑡 𝑑𝑡 (2)  ℎ 𝑡 𝑑𝑡 Acoustics 2020, 2 509 where h(t) is the impulse response; h10m is the impulse response at 10 m in a free field.  Figure 3. Room dimensions, source positions S1–S2 and receiver positions R1–R6. Figure 3. Room dimensions, source positions S1–S2 and receiver positions R1–R6.  In both speech clarity and sound strength, the early reflections are included. When evaluating T according to ISO 3382-2, the evaluation interval is 5 to 25 dB, given that the early reflections In both speech clarity and sound strength, the early reflections are included. When evaluating  20, are excluded. The evaluation concerns octave bands in range 125–4000 Hz averaged over source and T20, according to ISO 3382‐2, the evaluation interval is −5 to −25 dB, given that the early reflections are  microphone positions. excluded. The evaluation concerns octave bands in range 125–4000 Hz averaged over source and  The measurements were performed over the course of two days, with stable temperature microphone positions.  and humidity conditions. It was secured that there was no influence of background noise in The measurements were performed over the course of two days, with stable temperature and  the measurements. humidity  conditions.  It  was  secured  that  there  was  no  influence  of  background  noise  in  the  2.3. Repeatability Test of Measurement Method measurements.  A repeatability test for the measurement procedure used was performed. Impulse response measurements in the classroom mock-up, shown in Figure 2 were repeated five times. The room 2.3. Repeatability Test of Measurement Method  was furnished and had a suspended absorbent a ceiling. The practical absorption coecients for the ceiling are shown in Figure 4. Between each measurement, the equipment i.e., the loudspeaker A  repeatability  test  for  the  measurement  procedure  used  was  performed.  Impulse  response  and the microphone, was taken out from the room and reinstalled at di erent positions. Further, measurements in the classroom mock‐up, shown in Figure 2 were repeated five times. The room was  the loudspeaker was rotated, as this too can influence the measurements [37]. The measurement was furnished  and  had  a  suspended  absorbent  a  ceiling.  The  practical  absorption  coefficients  for  the  performed during the course of one day. Temperature and humidity were kept stable during the ceiling are shown in Figure 4. Between each measurement, the equipment i.e., the loudspeaker and  measurement procedures. However, with regards to the loudspeaker, it was always located at the front the microphone, was taken out from the room and reinstalled at different positions. Further, the  of the room in the vicinity of the teacher ’s desk. Two loudspeaker positions and six receiver positions for each loudspeaker position were used. Thus, a total of twelve observations were collected for each loudspeaker was rotated, as this too can influence the measurements [37]. The measurement was  measurement. The loudspeaker and microphone were always at least one meter from the surrounding performed during the course of one day. Temperature and humidity were kept stable during the  walls, and the receiver positions were no less than two meters from the loudspeaker. measurement procedures. However, with regards to the loudspeaker, it was always located at the  The purpose of the repeatability test was to establish the variation in the averaged room acoustical front of the room in the vicinity of the teacher’s desk. Two loudspeaker positions and six receiver  parameters reverberation time T , speech clarity C and sound strength G, when averaged over the 20 50 positions for each loudspeaker position were used. Thus, a total of twelve observations were collected  twelve combinations of loudspeaker and receiver positions. Knowing this variation gives an indication of the measurement procedure’s influence when comparing di erent scenarios of acoustical treatment for each measurement. The loudspeaker and microphone were always at least one meter from the  and the significance of the results. The spatial variation over positions, see Appendix A, are naturally surrounding walls, and the receiver positions were no less than two meters from the loudspeaker.  much larger than the variation of the averaged values in the repeatability test. It can, however, be noted Acoustics 2020, 2 510 that the spatial variation in rooms with ceiling treatment probably di er from what is expected under di use field conditions [38]. The repeatability test was performed for the basic configuration, i.e., the classroom with the furniture and ceiling treatment in Figure 4. The results of the five measurements of T , C and G are 20 50 presented in Table 1. The standard deviation for each pair of possible combinations, i.e., ten di erent combinations, from the five measurements was calculated and averaged. These results are also shown in Table 1 with the relative standard deviation is presented in the last column. Assuming normal distribution, an approximate uncertainty limit corresponding to a 95% confidence interval is presented in Table 2. Table 1. Results of repeatability test of the measurement method used in the study. Average values over the five di erent measurements, columns 2–6, average for each octave, column 7 and relative standard deviation, column 8. (a) Sound Strength G (dB) Test 1 2 3 4 5 G  /G ,avg avg ,avg 125 Hz 21.4 21.6 21.9 21.5 20.5 21.4 0.45 250 Hz 20.8 20.7 21.3 20.8 20.8 20.9 0.16 500 Hz 19.8 19.8 19.5 19.6 20.2 19.8 0.09 1000 Hz 19.3 19.0 19.0 19.2 19.5 19.2 0.04 2000 Hz 18.0 17.8 18.4 18.6 18.4 18.2 0.07 4000 Hz 18.0 17.7 18.5 18.2 18.4 18.2 0.05 (b) Speech Clarity C50 (dB) Test 1 2 3 4 5 C  /C 50.avg avg 50.avg 125 Hz 0.7 0.4 0.8 1.3 1.7 1.0 0.02 250 Hz 1.3 1.1 1.2 1.5 1.7 1.4 0.01 500 Hz 3.0 3.4 3.1 2,7 3.5 3.1 0.01 1000 Hz 4.6 4.4 4.0 4.4 4.2 4.3 0.01 2000 Hz 3.7 3.7 3.9 4.6 3.8 3.9 0.02 4000 Hz 5.0 4.4 5.3 5.1 5.0 5.0 0.01 (c) Reverberation Time T20 (s) Test 1 2 3 4 5 T  /T avg 20.avg 20.avg 125 Hz 1.50 1.43 1.44 1.40 1.56 1.47 0.038 250 Hz 1.45 1.43 1.47 1.44 1.45 1.45 0.009 500 Hz 0.81 0.81 0.82 0.82 0.83 0.82 0.009 1000 Hz 0.77 0.77 0.76 0.76 0.76 0.76 0.006 2000 Hz 0.91 0.89 0.89 0.91 0.90 0.90 0.008 4000 Hz 0.88 0.89 0.88 0.89 0.89 0.89 0.007 Table 2. Uncertainty interval related to repeatability, corresponding to a 95 % confidence interval, for the measurement procedure used in the experiments. G (dB) C (dB) T (s) avg 50,avg 20,avg 125 Hz 0.61 0.56 0.077 250 Hz 0.30 0.29 0.018 500 Hz 0.40 0.29 0.010 1000 Hz 0.25 0.27 0.006 2000 Hz 0.37 0.38 0.010 4000 Hz 0.36 0.36 0.008 It is concluded that the variations in repeated measurements are less than just noticeable di erences (JND), according to ISO 3382-1. This supports the discussion of significant di erences in the measurements. Acoustics 2020, 2 511 Acoustics 2020, 3 FOR PEER REVIEW   6  Figure 4. Absorption coecient for a 40 mm glass wool product used as absorbing material in the Figure 4. Absorption coefficient for a 40 mm glass wool product used as absorbing material in the  experiments. Blue: Absorption coecient for overall depth (ODS) 200. Red: ODS 50 mm. experiments. Blue: Absorption coefficient for overall depth (ODS) 200. Red: ODS 50 mm.  2.4. Acoustic Treatment The purpose of the repeatability test was to establish the variation in the averaged room acoustical  parameters reverberation time T20, speech clarity C50 and sound strength G, when averaged over the  2.4.1. Absorbing Material twelve  combinations  of  loudspeaker  and  receiver  positions.  Knowing  this  variation  gives  an  The absorbing material used in the form of ceiling panels and wall panels is a glass wool product indication  of  the  measurement  procedure’s  influence  when  comparing  different  scenarios  of  with a thickness of 40 mm and air flow resistivity of 40 kPa*s/m . The practical absorption coecient, acoustical  treatment  and  the  significance  of  the  results.  The  spatial  variation  over  positions,  see  , for the material according to ISO 11654 [39] can be seen in Figure 4 below. The absorption Appendix A, are naturally much larger than the variation of the averaged values in the repeatability  performance is shown for overall depth (ODS) 200 mm, according to specification in standard as well test. It can, however, be noted that the spatial variation in rooms with ceiling treatment probably  as for ODS of 50 mm, which represents the behavior of the material when mounted directly on the differ from what is expected under diffuse field conditions [38].  wall. This will be explained in a further section on configurations. The weighted absorption coecient The repeatability test was performed for the basic configuration, i.e., the classroom with the  is equal to 1 for both ODS set-ups. furniture and ceiling treatment in Figure 4. The results of the five measurements of T20, C50 and G are  For evaluation of the e ect of low frequency absorption, experiments were carried out with added presented in Table 1. The standard deviation for each pair of possible combinations, i.e., ten different  absorption on top of the suspended ceiling. The product used was a 50 mm glass wool product with combinations, from the five measurements was calculated and averaged. These results are also shown  air flow resistivity of 10 kPa*s/m encapsulated in a plastic foil. in Table 1 with the relative standard deviation is presented in the last column. Assuming normal  distribution,  an  approximate  uncertainty  limit  corresponding  to  a  95%  confidence  interval  is  2.4.2. Di users presented in Table 2.  The di users used were made of a wood frame with a surface of a curved hardboard. All di users It  is  concluded  that  the  variations  in  repeated  measurements  are  less  than  just  noticeable  tested had the same geometry and dimensions 600 mm 600 mm 100 mm, see Figure 5. Air gaps on differences (JND), according to ISO 3382‐1. This supports the discussion of significant differences in  the sides in combination with the enclosed volume gives the di user a Helmholtz resonance in the the measurements.  frequency range of 125–250 Hz. Acoustics 2020, 3 FOR PEER REVIEW   8  Table 1. Results of repeatability test of the measurement method used in the study. Average values  over the five different measurements, columns 2–6, average for each octave, column 7 and relative  standard deviation, column 8.  (a) Sound Strength G (dB)  Test  1  2  3  4  5  G,avg σavg/G,avg  125 Hz  21.4  21.6  21.9  21.5  20.5  21.4  0.45  250 Hz  20.8  20.7  21.3  20.8  20.8  20.9  0.16  500 Hz  19.8  19.8  19.5  19.6  20.2  19.8  0.09  1000 Hz  19.3  19.0  19.0  19.2  19.5  19.2  0.04  2000 Hz  18.0  17.8  18.4  18.6  18.4  18.2  0.07  4000 Hz  18.0  17.7  18.5  18.2  18.4  18.2  0.05  Figure 5. Sketch of diffuser used in the study, horizontally oriented.  Figure 5. Sketch of di user used in the study, horizontally oriented. (b) Speech Clarity C50 (dB)  Test  1  2  3  4  5  C50.avg σavg/C50.avg  Diffusion  characteristics  were  measured  in  a  semi‐anechoic  chamber.  The  energy  in  the  125 Hz −0.7 −0.4 −0.8 −1.3 −1.7 −1.0 −0.02  reflections were estimated from impulse responses using windowing techniques, excluding the direct  sound. The reflections were measured for azimuthal angles (θ) 0–90 degrees. Figure 6 presents the  diffusion characteristics for 500, 2000 and 4000 Hz, and the assumption of symmetrical properties has  been applied.  (a)  (b)     (c)  (d)  Figure 6. Diffusion characteristics at (b) 500, (c) 2000 and (d) 4000 Hz. The upper left figure (a) shows  the orientation of the diffusers relative room coordinates, see Figure 1.  The diffusers were tested in a vertical and a horizontal direction. In the vertical the majority of  sound waves were directed in z‐direction, Figure 7. while for horizontal is the majority of waves  directed in x‐y plane, Figure 8.  Acoustics 2020, 3 FOR PEER REVIEW   8  Figure 5. Sketch of diffuser used in the study, horizontally oriented.  Acoustics 2020, 2 512 Diffusion  characteristics  were  measured  in  a  semi‐anechoic  chamber.  The  energy  in  the  reflections were estimated from impulse responses using windowing techniques, excluding the direct  Di usion characteristics were measured in a semi-anechoic chamber. The energy in the reflections sound. The reflections were measured for azimuthal angles (θ) 0–90 degrees. Figure 6 presents the  were estimated from impulse responses using windowing techniques, excluding the direct sound. diffusion characteristics for 500, 2000 and 4000 Hz, and the assumption of symmetrical properties has  The reflections were measured for azimuthal angles () 0–90 degrees. Figure 6 presents the di usion been applied.  characteristics for 500, 2000 and 4000 Hz, and the assumption of symmetrical properties has been applied. (a)  (b)     (c)  (d)  Figure Figure  6. Diffusion 6. Di usion  characteristics characteristics atat (b (b) )500, 500, ((c c)) 2000 2000 and and( ( dd ))4000  4000 Hz.  Hz. The  The upper  upper left left figur fiegure (a) shows  (a) shows  the orientation of the di users relative room coordinates, see Figure 1. the orientation of the diffusers relative room coordinates, see Figure 1.  The di users were tested in a vertical and a horizontal direction. In the vertical the majority of The diffusers were tested in a vertical and a horizontal direction. In the vertical the majority of  sound waves were directed in z-direction, Figure 7. while for horizontal is the majority of waves sound waves were directed in z‐direction, Figure 7. while for horizontal is the majority of waves  directed in x-y plane, Figure 8. directed in x‐y plane, Figure 8.  Acoustics 2020, 2 513 Acoustics 2020, 3 FOR PEER REVIEW   9  Acoustics 2020, 3 FOR PEER REVIEW   9  Figure 7. Vertically oriented diffusers, majority of reflections will be sent in z‐direction.  Figure 7. Vertically oriented diffusers, majority of reflections will be sent in z‐direction.  Figure 7. Vertically oriented di users, majority of reflections will be sent in z-direction. Figure 8. Horizontally oriented diffusers, majority of reflections will be sent in x‐direction.  Figure 8. Horizontally oriented di users, majority of reflections will be sent in x-direction. Figure 8. Horizontally oriented diffusers, majority of reflections will be sent in x‐direction.  2.5. Configurations 2.5. Configurations  2.5. Configurations  For this experimental series, nine di erent configurations were tested, starting from the empty For this experimental series, nine different configurations were tested, starting from the empty  For this experimental series, nine different configurations were tested, starting from the empty  room. Thereafter, there was an absorbent ceiling, with properties according to Section 2.4.1. added room. Thereafter, there was an absorbent ceiling, with properties according to Section 2.4.1. added  room. Thereafter, there was an absorbent ceiling, with properties according to Section 2.4.1. added  and further was the room furnished. From this configuration was di erent type of wall treatment and further was the room furnished. From this configuration was different type of wall treatment  and further was the room furnished. From this configuration was different type of wall treatment  added, three di erent configurations: absorbing material, according to Section 2.4.1; vertically oriented added,  three  different  configurations:  absorbing  material,  according  to  Section  2.4.1;  vertically  added,  three  different  configurations:  absorbing  material,  according  to  Section  2.4.1;  vertically  di users, as in Figure 7; and horizontally oriented di users, as in Figure 8. Additional di users were oriented  diffusers,  as  in  Figure  7;  and  horizontally  oriented  diffusers,  as  in  Figure  8.  Additional  oriented  diffusers,  as  in  Figure  7;  and  horizontally  oriented  diffusers,  as  in  Figure  8.  Additional  installed in the ceiling. These di users were located in the front area of the room, i.e., in a typical diffusers were installed in the ceiling. These diffusers were located in the front area of the room, i.e.,  diffusers were installed in the ceiling. These diffusers were located in the front area of the room, i.e.,  speaker position. In the last configurations, low frequency absorption was investigated; Helmholtz in a typical speaker position. In the last configurations, low frequency absorption was investigated;  in a typical speaker position. In the last configurations, low frequency absorption was investigated;  resonance absorption by di users, compared to a porous absorber with properties for good absorption Helmholtz resonance absorption by diffusers, compared to a porous absorber with properties for  Helmholtz resonance absorption by diffusers, compared to a porous absorber with properties for  properties in this frequency range (see the last section of Section 2.4.1). All configurations are described good  absorption  properties  in  this  frequency  range  (see  the  last  section  of  Section  2.4.1).  All  good  absorption  properties  in  this  frequency  range  (see  the  last  section  of  Section  2.4.1).  All  in Table 3. For full abbreviations and definitions see Abbreviations. configurations are described in Table 3. For full abbreviations and definitions see Abbreviations.  configurations are described in Table 3. For full abbreviations and definitions see Abbreviations.  Acoustics 2020, 2 514 Table 3. Configurations in the test series: Configuration number, definition with abbreviations and description of the configurations. Configuration Configuration Definition Configuration Description 1 Empty No acoustic treatment 2 51.8CA 51.8 m absorptive ceiling 3 51.8CA_F (2) + furniture 4 51.8CA_F_8.64WA (3) + 8.6 m wall absorbers 5 51.8CA_F_8.64VWD (3) + 8.6 m vertical wall di users 6 51.8CA_F_8.64HWD (3) + 8.6 m horizontal wall di users 2 2 47.5 m absorptive ceiling, 4.3 m ceiling 7 47.5CA_4.3CD_F_ 8.6WA di users, furniture and 8.6 m wall absorbers 8 51.8CA_8.6VWD (2) + 8.64 m vertical wall di users (2) + 25.0 m low frequency absorptive 9 51.8CA_25.0LFMA mineral wool added in the ceiling 3. Results The following section is divided into four di erent subsections, presenting the room acoustic parameters for di erent configurations. Section 3.1. represents the e ect of traditional acoustic treatment and furniture. Section 3.2. includes the e ect of acoustic treatment, absorbers and di users, on the walls, using the di users in di erent orientations. Section 3.3 describes how the di users were placed on the ceiling and Section 3.4. includes the e ect of additional low frequency absorption. The results are presented in the form of diagrams, evaluated over octave frequency bands. All values for the room acoustic parameters with the corresponding standard deviation are presented in Appendix A. 3.1. E ect of Absorbent Ceiling and Furniture The graphs presented in Figure 9 show the room acoustic parameters for a room without treatment (Empty), a room with absorbent ceiling (51.8CA) and a room with absorbent ceiling and furniture (51.8CA_F). For description of furnishing see Section 2.1. Room mock-up. Comparing the empty room with a configuration using acoustic ceiling shows a clear di erence for all acoustic parameters over the entire frequency range, with the strongest change from 500 Hz and upwards, which can be correlated to the acoustic performance of the ceiling (see Figure 4). The sound strength in Figure 9a decreases by as much as 8 dB at these frequencies, and speech clarity, in Figure 9b, by 7 dB. The reverberation time, in Figure 9c, decreases to approximately half the value. The reason for the short reverberation times at low frequencies, in an already empty room, is due to the fact that the surrounding walls in the classroom were lightweight walls of plaster board. Adding furniture contributes by scattering the sound and is e ective from 500–2000 Hz, with the largest di erences at 500–1000 Hz, resulting in an additional change in curve shapes for the room acoustic parameters. The sound strength value is mainly dependent on the absorption area, but a decrease of about 1 dB for the frequencies 500–4000 Hz can still be found for this parameter, due to scattering and minor absorption from the upholstered chairs. It should be noted that 1 dB is considered a just noticeable di erence (JND) for sound strength, according to ISO 3382-1 [24]. The speech clarity and reverberation time is a ected in a more limited frequency range, 500–1000 Hz, with significant di erences. Regarding speech clarity, an increase of 3 dB is achieved at 500 Hz, and 2 dB at 1000 Hz. As for sound strength, 1 dB di erence in speech clarity is considered to be JND, according to [24]. The reverberation time decreased by nearly half at 500 Hz, from 1.5 s to 0.8 s, with less reduction at 1000 Hz and 2000 Hz, although still a noticeable di erence, a decrease of 0.6 s at 1000 Hz, and 0.3 s at 2000 Hz. JND for a reverberation time is a change of 5% [24]. Acoustics 2020, 3 FOR PEER REVIEW   11  Acoustics 2020, 3 FOR PEER REVIEW   11  Acoustics 2020, 2 515 Figure  9.  Room  acoustic  parameters,  (a)  sound  strength  (G),  (b)  speech  clarity  (C50)  and  (c)  reverberation time (T20). In blue: room with no treatment (Empty). Red: room with absorbent ceiling   Figure 9. Room acoustic parameters, (a) sound strength (G), (b) speech clarity (C ) and (c) reverberation (51.8CA). Green: room with absorbent ceiling and furniture (51.8CA_F). Installation of ceiling gives  Figure  9.  Room  acoustic  parameters,  (a)  sound  strength  (G),  (b)  speech  clarity  (C50)  and  (c)  time (T ). In blue: room with no treatment (Empty). Red: room with absorbent ceiling (51.8CA). Green: solidreverberation  differences  for time the  (T 20entir ). In eblue:  frequency  room with  range  no  while treatment  furniture  (Empty).  has Red:  highest  room eff  with icien  abso cyrbent  at 500–2000  ceiling  Hz.  room with absorbent ceiling and furniture (51.8CA_F). Installation of ceiling gives solid di erences for (51.8CA). Green: room with absorbent ceiling and furniture (51.8CA_F). Installation of ceiling gives  All parameters are affected by the furniture; thus, the greatest differences are seen for C50 and T20.  the entire frequency range while furniture has highest eciency at 500–2000 Hz. All parameters are solid differences for the entire frequency range while furniture has highest efficiency at 500–2000 Hz.  a ected by the furniture; thus, the greatest di erences are seen for C and T . 50 20 All parameters are affected by the furniture; thus, the greatest differences are seen for C50 and T20.  Comparing  the  measured  value  to  calculation  with  assumption  of  diffuse  sound  field  using  Comparing the measured value to calculation with assumption of di use sound field using Sabine’s formula shows substantially lower values for calculation over the entire frequency range  Comparing  the  measured  value  to  calculation  with  assumption  of  diffuse  sound  field  using  Sabine’s formula shows substantially lower values for calculation over the entire frequency range compared to the measurement, see Figure 10. Note also that the shape of the two curves differs. The  Sabine’s formula shows substantially lower values for calculation over the entire frequency range  compared to the measurement, see Figure 10. Note also that the shape of the two curves di ers. slight  increase  at  higher  frequencies  in  the  measured  curve  often  appears  in  sparsely  furnished  compared to the measurement, see Figure 10. Note also that the shape of the two curves differs. The  The slight increase at higher frequencies in the measured curve often appears in sparsely furnished rooms, due to the lateral reflections from walls. The frequency‐dependent effect of scattering is not  slight  increase  at  higher  frequencies  in  the  measured  curve  often  appears  in  sparsely  furnished  rooms, due to the lateral reflections from walls. The frequency-dependent e ect of scattering is not included  in  the  Sabine  calculation,  but  appears  as  a  valley  in  the  measured  curve.  Measured  rooms, due to the lateral reflections from walls. The frequency‐dependent effect of scattering is not  included in the Sabine calculation, but appears as a valley in the measured curve. Measured absorption absorption from furniture is taken into account in the calculation.  included  in  the  Sabine  calculation,  but  appears  as  a  valley  in  the  measured  curve.  Measured  from furniture is taken into account in the calculation. absorption from furniture is taken into account in the calculation.  Figure 10. In blue measured and in red calculated reverberation time for the room with absorbent  Figure 10. In blue measured and in red calculated reverberation time for the room with absorbent Figure 10. In blue measured and in red calculated reverberation time for the room with absorbent  ceiling and furniture.  ceiling and furniture. ceiling and furniture.  3.2. The E ect of Acoustic Treatments on the Walls 3.2. The Effect of Acoustic Treatments on the Walls  3.2. The Effect of Acoustic Treatments on the Walls  The following graphs in Figure 11. present the room acoustic parameters for configurations The following graphs in Figure 11. present the room acoustic parameters for configurations with  The following graphs in Figure 11. present the room acoustic parameters for configurations with  with acoustic treatment placed on the adjacent walls. Two walls are used, 4.3 2 m is covered with acoustic  treatment  placed  on  the  adjacent  walls.  Two  walls  are  used,  4.3  m   is  covered  with  the  acou the stictr  trea eatment tment on  pla each ced wall   on for theall  adjac three ent configurations   walls.  Two  pr walls esented   are in used, this  section, 4.3  m   is i.e.,  covered coverage  wit 2 ofh  the  treatment on each wall for all three configurations presented in this section, i.e., coverage of 8.6 m  in  8.6 m in total. The configurations are with wall absorption (51.8CA_F_8.6WA), vertically oriented treatment on each wall for all three configurations presented in this section, i.e., coverage of 8.6 m  in  total.  The  configurations  are  with  wall  absorption  (51.8CA_F_8.6WA),  vertically  oriented  wall  wall di users (51.8CA_F_8.6VWD) and horizontally oriented wall di users (51.8CA_F_8.6HWD) diffusers  (51.8CA_F_8.6VWD)  and  horizontally  oriented  wall  diffusers  (51.8CA_F_8.6HWD)  (see  total.  The  configurations  are  with  wall  absorption  (51.8CA_F_8.6WA),  vertically  oriented  wall  (see Figures 7 and 8). For all configurations in this section, a full covering absorbent ceiling is installed Figure  7  and  Figure  8).  For  all  configurations  in  this  section,  a  full  covering  absorbent  ceiling  is  diffusers  (51.8CA_F_8.6VWD)  and  horizontally  oriented  wall  diffusers  (51.8CA_F_8.6HWD)  (see  and the room is sparsely furnished, as in configuration 51.8CA_F, which is also included in graphs installed and the room is sparsely furnished, as in configuration 51.8CA_F, which is also included in  Figure  7  and  Figure  8).  For  all  configurations  in  this  section,  a  full  covering  absorbent  ceiling  is  below for comparison. graphs below for comparison.  installed and the room is sparsely furnished, as in configuration 51.8CA_F, which is also included in  graphs below for comparison.  Acoustics 2020, 3 FOR PEER REVIEW   12  Acoustics 2020, 2 516 Figure 11. Room acoustic parameters, (a) sound strength (G), (b) speech clarity (C ) and (c) reverberation time (T ). Blue: base configuration with only ceiling treatment and furniture (51.8CA_F) Figure  11.  Room  acoustic  parameters,  (a)  sound  strength  (G),  (b)  speech  clarity  (C50)  and  (c)  to be compared with configurations having acoustic treatment on the walls. Red: porous absorbers reverberation  time  (T20).  Blue:  base  configuration  with  only  ceiling  treatment  and  furniture  (51.8CA_F_8.6WA). Green: vertically directed di users (51.8CA_F_8.6VWD). Violet: horizontally (51.8CA_F) to be compared with configurations having acoustic treatment on the walls. Red: porous  directed di users (51.8CA_F_8.6HWD). The di erent types of acoustic wall treatment cover the same absorbers  (51.8CA_F_8.6WA).  Green:  vertically  directed  diffusers  (51.8CA_F_8.6VWD).  Violet:  area in all three cases. The strongest impact on G is achieved with wall absorbers. Di users also had a horizontally directed diffusers (51.8CA_F_8.6HWD). The different types of acoustic wall treatment  minor e ect on this parameter, with similar values obtained independent of orientation. For C and cover the same area in all three cases. The strongest impact on G is achieved with wall absorbers.  T , the orientation of di users is critical, with a greater e ect achieved by vertically directed di users. Diffusers  also had  a  minor effect  on  this  parameter,  with similar values  obtained independent  of  The configuration with wall absorbers results in lower sound strength values in a frequency range of orientation. For C50 and T20, the orientation of diffusers is critical, with a greater effect achieved by  250 Hz to 4000 Hz, both in comparison with di user configurations, as well as with configurations with vertically directed diffusers.  no wall treatment; a decrease is seen for the entire frequency range in this comparison. The di erences are small, The configuration but a clear tr with end iswa ap ll par absorb ent ers (see results the graph  in loin wer Figur  soun e d 11 st ).rength Up to values 0.8 dB, inlower  a freq values uency  ar range e obtained   of  250 for Hzthe   to configur 4000  Hz, ation   both with   in  comparison absorbing wall   with tr eatment diffuser and conf the igur diat io using ns,  as wall   well tr eatment as  with  configurations  with  no  wall  treatment;  a  decrease  is  seen  for  the  entire  frequency  range  in  this  within the frequency range of 250–4000 Hz. Thus, G is still lower for configurations with di users, compar comparison ed to. The no wall  differenc treatment es are(51.8CA_F).  small, but a Further clear trend , a simil  is appa ar Grent is obtained  (see the for graph configurations  in Figure 11). with  Up  to 0.8 dB, lower values are obtained for the configuration with absorbing wall treatment and the  di user treatment (51.8CA_F_8.6VWD) and (51.8CA_F_8.6HWD), i.e., the values for this parameter diffusing  wall  treatment  within  the  frequency  range  of  250–4000  Hz.  Thus,  G  is  still  lower  for  are independent of the direction of di users. Note also from this graph, that the lower values in G for configurations  with  diffusers,  compared  to  no  wall  treatment  (51.8CA_F).  Further,  a  similar  G  is  configurations with di users is at a frequency of 125 Hz. This decrease is not correlated to scattering, obtained for configurations with diffuser treatment (51.8CA_F_8.6VWD) and (51.8CA_F_8.6HWD),  but it is due to the resonance absorption for this frequency included in the design of the di users used i.e., the values for this parameter are independent of the direction of diffusers. Note also from this  in the study. This is shown further in Section 3.4. graph, that the lower values in G for configurations with diffusers is at a frequency of 125 Hz. This  Speech clarity increases for all configurations with any type of wall treatment. The largest decrease is not correlated to scattering, but it is due to the resonance absorption for this frequency  increase is seen for configuration with wall absorbers (51.8CA_8.6WA) at a frequency of 500–4000 Hz. included in the design of the diffusers used in the study. This is shown further in Section 3.4.  In comparison with di users, the change is largest at 4000 Hz, with a 1.7 dB and 2.8 dB di erence for Speech  clarity  increases  for  all  configurations  with  any  type  of  wall  treatment.  The  largest  vertically oriented and horizontally oriented, respectively. increase is seen for configuration with wall absorbers (51.8CA_8.6WA) at a frequency of 500–4000  The two configurations with di users have similar C values in octave bands 125 Hz to 500 Hz, Hz. In comparison with diffusers, the change is largest at 4000 Hz, with a 1.7 dB and 2.8 dB difference  i.e., in the range where these di users are not designed to be e ective. However, at 1000 Hz to 4000 Hz, for vertically oriented and horizontally oriented, respectively.  clearly higher C values are obtained for the vertically oriented di users—about 0.8–1.0 dB higher, The two configurations with diffusers have similar C50 values in octave bands 125 Hz to 500 Hz,  compared to horizontally oriented di users. This is a frequency range where the di users are e ective, i.e., in the range where these diffusers are not designed to be effective. However, at 1000 Hz to 4000  but the vertically oriented di users, to a greater degree, distract the lateral sound field and redirect the Hz,  clearly  higher  C50  values  are  obtained  for  the  vertically  oriented  diffusers—about  0.8–1.0  dB  sound to the absorbent ceiling. The same behavior between the di erent configurations is seen for higher, compared to horizontally oriented diffusers. This is a frequency range where the diffusers are  reverberation time. In addition to this, for the higher frequencies, similar results are achieved in T effective, but the vertically oriented diffusers, to a greater degree, distract the lateral sound field and  for configuration with vertical di users and wall absorbers. A change in the behavior is obtained for redirect the sound to the absorbent ceiling. The same behavior between the different configurations  frequency 125 Hz for both C and T in configurations with di users. As for sound strength, this is 50 20 is  seen  for  reverberation  time.  In  addition  to  this,  for  the  higher  frequencies,  similar  results  are  due to resonance absorption in the di users. achieved in T20 for configuration with vertical diffusers and wall absorbers. A change in the behavior  is obtained for frequency 125 Hz for both C50 and T20 in configurations with diffusers. As for sound  strength, this is due to resonance absorption in the diffusers.  Acoustics 2020, 2 517 Acoustics 2020, 3 FOR PEER REVIEW   13  Acoustics 2020, 3 FOR PEER REVIEW   13  3.3. Ceiling Di users 3.3. Ceiling Diffusers  3.3. Ceiling Diffusers  The following section presents the e ect of di users installed in the ceiling. Six of the absorbent The following section presents the effect of diffusers installed in the ceiling. Six of the absorbent  The following section presents the effect of diffusers installed in the ceiling. Six of the absorbent  panels in the front of the room, typical speaker position, were replaced with di users, corresponding panels in the front of the room, typical speaker position, were replaced with diffusers, corresponding  panels in 2  the front of the room, typical speaker position, were 2  replaced with diffusers, corresponding  to 4.3 m , see Figure 12. The walls are covered with 8.6 m of absorbers and the room is sparsely 2 2 to 4.3 m , see Figure 12. The walls are covered with 8.6 m  of absorbers and the room is sparsely  2 2 to 4.3 m , see Figure 12. The walls are covered with 8.6 m  of absorbers and the room is sparsely  furnished (47.5CA_4.3CD_F_8.6WA). furnished (47.5CA_4.3CD_F_8.6WA).  furnished (47.5CA_4.3CD_F_8.6WA).  Figure 12. Location of ceiling diffusers above assumed speaker position. R2 corresponds to position  Figure 12. Location of ceiling diffusers above assumed speaker position. R2 corresponds to position  Figure 12. Location of ceiling di users above assumed speaker position. R2 corresponds to position for listener close to the speaker, R5 corresponds to listener in the rear area of the room.  for listener close to the speaker, R5 corresponds to listener in the rear area of the room.  for listener close to the speaker, R5 corresponds to listener in the rear area of the room. The results for the average values over the twelve measurements show a general decrease in G  The The r esults results for fothe r the average  averag ve alues  values over over the th twelve e twelmeasur ve measurements ements show show a general  a gener decr al ease decre inase G and  in G  and T20, and an increase of C50. The difference is small but a clear trend is obtained, see Figure 13.  Tand , and  T20,an and incr an ease  incre of a Cse .of The  C50di . The erence  difference is small isbut  smaall clear  but tra end cleais r trend obtained,  is obtained, see Figur see e 13 Figure . Ceiling  13.  20 50 Ceiling  diffuser  configurations  were  also  tested  with  no  wall  treatment  or  wall  diffusers,  with  diCeil user ingconfigurations   diffuser  config wer ureaalso tions tested   werewith   alsono  tested wall trwi eatment th  no  or wall wall   trea di tment users,  or with   wall equivalent   diffusertr s,ends   with  equivalent trends being obtained (for the results, see Appendix A).  being equivalent obtained  trends (for being the results,  obtained see Appendix (for the results, A).  see Appendix A).  Figure 13. Global figures, averaged over all source and receiver positions. In (a) sound strength (G), Figure 13. Global figures, averaged over all source and receiver positions. In (a) sound strength (G),  Figure 13. Global figures, averaged over all source and receiver positions. In (a) sound strength (G),  (b) speech clarity (C ) and (c) reverberation time (T ). Blue: room with absorbent ceiling, furniture 50 20 (b) speech clarity (C50) and (c) reverberation time (T20). Blue: room with absorbent ceiling, furniture  (b) speech clarity (C50) and (c) reverberation time (T20). Blue: room with absorbent ceiling, furniture  and wall absorbers (51.8CA_F_8.6WA). Red: room with partly absorbent ceiling, partly ceiling di users, and  wall  absorbers  (51.8CA_F_8.6WA).  Red:  room  with  partly  absorbent  ceiling,  partly  ceiling  and  wall  absorbers  (51.8CA_F_8.6WA).  Red:  room  with  partly  absorbent  ceiling,  partly  ceiling  furniture and wall absorbers (47.5CA_4.3CD_F_8.6WA). Configuration with ceiling di users gives diffusers,  furniture  and  wall  absorbers  (47.5CA_4.3CD_F_8.6WA).  Configuration  with  ceiling  diffusers,  furniture  and  wall  absorbers  (47.5CA_4.3CD_F_8.6WA).  Configuration  with  ceiling  lower value in G, with a clear trend apparent even if the di erence is small. In terms of C ceiling diffusers gives lower value in G, with a clear trend apparent even if the difference is small. In terms  diffusers gives lower value in G, with a clear trend apparent even if the difference is small. In terms  di users give an increase from frequency 500 Hz and upwards, with the greatest di erence being of  C50  ceiling  diffusers  give  an  increase  from  frequency  500  Hz  and  upwards,  with  the  greatest  of  C50  ceiling  diffusers  give  an  increase  from  frequency  500  Hz  and  upwards,  with  the  greatest  0.8 dB, at 1000 Hz. The reverberation time decreased in configuration with ceiling di users over the difference being 0.8 dB, at 1000 Hz. The reverberation time decreased in configuration with ceiling  difference being 0.8 dB, at 1000 Hz. The reverberation time decreased in configuration with ceiling  entire frequency range, with the change being small but the trend clear. diffusers over the entire frequency range, with the change being small but the trend clear.  diffusers over the entire frequency range, with the change being small but the trend clear.  The e ect of ceiling di users was further evaluated for di erent positions in the room by comparing The  effect  of  ceiling  diffusers  was  further  evaluated  for  different  positions  in  the  room  by  The  effect  of  ceiling  diffusers  was  further  evaluated  for  different  positions  in  the  room  by  the room acoustic parameters for receiver positions R2 and R5, source position S2. Positions are comparing  the  room  acoustic  parameters  for  receiver  positions  R2  and  R5,  source  position  S2.  comparing  the  room  acoustic  parameters  for  receiver  positions  R2  and  R5,  source  position  S2.  described in Figure 3 and further visualized in Figure 12. Positions are described in Figure 3 and further visualized in Figure 12.  Positions are described in Figure 3 and further visualized in Figure 12.  Sound strength decreases with distance from the source, but a comparison to 51.8CA_F_8.6WA  Sound strength decreases with distance from the source, but a comparison to 51.8CA_F_8.6WA  show higher values for configuration with ceiling diffusers 47.5CA_4.3CD_F_8.6WA. In addition to  show higher values for configuration with ceiling diffusers 47.5CA_4.3CD_F_8.6WA. In addition to  Acoustics 2020, 2 518 Acoustics 2020, 3 FOR PEER REVIEW   14  Sound strength decreases with distance from the source, but a comparison to 51.8CA_F_8.6WA Acoustics 2020, 3 FOR PEER REVIEW   14  the increased energy level, a significant increase is obtained for C50. In R5, i.e., in the back of the room,  show higher values for configuration with ceiling di users 47.5CA_4.3CD_F_8.6WA. In addition to the a difference can be seen for the entire frequency range, with strongest effect at 1000–4000 Hz, an  the increased energy level, a significant increase is obtained for C50. In R5, i.e., in the back of the room,  increased energy level, a significant increase is obtained for C . In R5, i.e., in the back of the room, a increase of 1.5–3.2 dB. The reverberation time is also affected, mainly in the higher frequency range.  a difference can be seen for the entire frequency range, with strongest effect at 1000–4000 Hz, an  di erence can be seen for the entire frequency range, with strongest e ect at 1000–4000 Hz, an increase Results are shown in Figure 14 and Figure 15.  increase of 1.5–3.2 dB. The reverberation time is also affected, mainly in the higher frequency range.  of 1.5–3.2 dB. The reverberation time is also a ected, mainly in the higher frequency range. Results are Results are shown in Figure 14 and Figure 15.  shown in Figures 14 and 15. Figure 14. In (a) sound strength (G), (b) speech clarity (C50) and (c) reverberation time (T20) in position  R2.  Blue:  without  ceiling  diffusers  (51.8CA_F_8.6WA).  Red:  with  ceiling  diffusers  Figure 14. In (a) sound strength (G), (b) speech clarity (C ) and (c) reverberation time (T ) in position R2. 50 20 Figure 14. In (a) sound strength (G), (b) speech clarity (C50) and (c) reverberation time (T20) in position  (47.5CA_4.3CD_F_8.6WA). Small increases in G and C50 are achieved in configurations with diffusers  Blue: without ceiling di users (51.8CA_F_8.6WA). Red: with ceiling di users (47.5CA_4.3CD_F_8.6WA). R2.  Blue:  without  ceiling  diffusers  (51.8CA_F_8.6WA).  Red:  with  ceiling  diffusers  in combinations with slightly lower T20.  Small increases in G and C are achieved in configurations with di users in combinations with slightly (47.5CA_4.3CD_F_8.6WA). Small increases in G and C50 are achieved in configurations with diffusers  lower T . in combinations with slightly lower T20.  Figure 15. In (a) sound strength (G), (b) speech clarity (C ) and (c) reverberation time (T ) in position 50 20 Figure 15. In (a) sound strength (G), (b) speech clarity (C50) and (c) reverberation time (T20) in position  R5, back of the room. Blue: without ceiling di users (51.8CA_F_8.6WA). Red: with ceiling di users R5, back of the room. Blue: without ceiling diffusers (51.8CA_F_8.6WA). Red: with ceiling diffusers  Figure 15. In (a) sound strength (G), (b) speech clarity (C50) and (c) reverberation time (T20) in position  (47.5CA_4.3CD_F_8.6WA). A small increase is seen for G. A significant increase in C is achieved with (47.5CA_4.3CD_F_8.6WA). A small increase is seen for G. A significant increase in C50 is achieved  R5, back of the room. Blue: without ceiling diffusers (51.8CA_F_8.6WA). Red: with ceiling diffusers  slightly lower T . The strongest improvements are seen in position R5 and in the frequency range of with slightly lower T20. The strongest improvements are seen in position R5 and in the frequency  (47.5CA_4.3CD_F_8.6WA). A small increase is seen for G. A significant increase in C50 is achieved  1000–4000 Hz. range of 1000—4000 Hz.  with slightly lower T20. The strongest improvements are seen in position R5 and in the frequency  3.4. Combining Di users with Helmholtz Absorption range of 1000—4000 Hz.  3.4. Combining Diffusers with Helmholtz Absorption  One way to obtain good absorption in a certain frequency is to use resonance absorbers. 3.4. Combining One way  Diffusers to obtain wi good th Helmh  absorp oltzt iAbsorption on in a certain    frequency is to use resonance absorbers. The  The di users used in this study were designed to operate as Helmholtz resonator at frequencies diffusers used in this study were designed to operate as Helmholtz resonator at frequencies 125–250  One way to obtain good absorption in a certain frequency is to use resonance absorbers. The  125–250 Hz. The result of its e ect in configuration 51.8CA_F_8.6VWD is compared with configuration Hz. The result of its effect in configuration 51.8CA_F_8.6VWD is compared with configuration with  with diffusers a por use ousd low in thi frsequency  study were absorber  desig,ned configuration  to operate as 51.8CA_25.0LFMA,  Helmholtz resonadescribed tor at frequenc in Section ies 125–2 2.4.1 50.  a  porous  low  frequency  absorber,  configuration  51.8CA_25.0LFMA,  described  in  Section  2.4.1.  Hz. The result of its effect in configuration 51.8CA_F_8.6VWD is compared with configuration with  Evaluation of reverberation time shows a clear e ect for both configurations at low frequencies. Evaluation of reverberation time shows a clear effect for both configurations at low frequencies. It  It a could porous be  lo noted w  fre that quency 8.6 m  ab diso rb users er,  co wer nfig e used uration while   51the .8CA ar_2 ea5. of0L por FMA, ous  low described frequency   in  Section absorber  2.4.1. was  could be noted that 8.6 m  diffusers were used while the area of porous low frequency absorber was  Evaluation of reverberation time shows a clear effect for both configurations at low frequencies. It  25 m . Additionally, the diffusers affect the higher frequency range to a greater extent. The results are  could be noted that 8.6 m  diffusers were used while the area of porous low frequency absorber was  presented in Figure 16.  25 m . Additionally, the diffusers affect the higher frequency range to a greater extent. The results are  presented in Figure 16.  Acoustics 2020, 2 519 25 m . Additionally, the di users a ect the higher frequency range to a greater extent. The results are Acoustics 2020, 3 FOR PEER REVIEW   15  presented in Figure 16. Figure 16. Reverberation time (T ), in blue only absorbent ceiling (51.8CA), in green vertical wall Figure 16. Reverberation time (T20), in blue only absorbent ceiling (51.8CA), in green vertical wall  di users (51.8CA_8.6VWD) and in red low frequency porous absorber (51.8CA_25.0LFMA). The two diffusers (51.8CA_8.6VWD) and in red low frequency porous absorber (51.8CA_25.0LFMA). The two  latter configurations a ect the lower frequencies significantly; thus, the areas used for the di erent latter configurations affect the lower frequencies significantly; thus, the areas used for the different  low frequency treatments are di erent. In addition, the di users a ect the higher frequency range to a low frequency treatments are different. In addition, the diffusers affect the higher frequency range to  greater extent. a greater extent.  4. Discussion 4. Discussion  Installing the fully covering absorbent ceiling in the empty room, as a first step, a ected all the room acoustic parameters. This treatment can be seen as a good baseline for a classroom, since it Installing the fully covering absorbent ceiling in the empty room, as a first step, affected all the  significantly decreases the sound strength and reverberation time, and increases the speech clarity. room acoustic parameters. This treatment can be seen as a good baseline for a classroom, since it  The addition of furniture also a ected the results, mainly due to increased scattering, as only a small significantly decreases the sound strength and reverberation time, and increases the speech clarity.  amount of absorption is involved. The furniture a ects the parameters particularly at frequencies of The addition of furniture also affected the results, mainly due to increased scattering, as only a small  500–1000 Hz. This configuration, an absorbent ceiling and a sparsely furnished room, could be seen as amount of absorption is involved. The furniture affects the parameters particularly at frequencies of  a normal classroom situation. It is important to note the di erence in result between measurement 500–1000 Hz. This configuration, an absorbent ceiling and a sparsely furnished room, could be seen  and calculated T by using di use sound field theory for this configuration. The much lower values achieved in calculation demonstrate that the lateral sound field must be considered in acoustic models as a normal classroom situation. It is important to note the difference in result between measurement  dealing with ordinary, furnished rooms. and calculated T20 by using diffuse sound field theory for this configuration. The much lower values  The additional acoustic treatment in di erent configurations using absorbers and di users achieved  in  calculation  demonstrate  that  the  lateral  sound  field  must  be  considered  in  acoustic  contributed important e ects for fine-tuning the acoustics. The general finding was that higher models dealing with ordinary, furnished rooms.  sound energy levels were obtained for configurations with di users, and lower energy levels for The  additional  acoustic  treatment  in  different  configurations  using  absorbers  and  diffusers  configurations with absorbers. Both types of treatment a ected speech clarity and reverberation time. contributed important effects for fine‐tuning the acoustics. The general finding was that higher sound  With the di users, the energy is conserved, compared to the absorbers where the energy is energy  levels  were  obtained  for  configurations  with  diffusers,  and  lower  energy  levels  for  reduced, explaining why both horizontally oriented and vertically oriented di users have similar configurations with absorbers. Both types of treatment affected speech clarity and reverberation time.  values in terms of sound strength. A reason for the e ect on speech clarity and reverberation time is With  the  diffusers,  the  energy  is  conserved,  compared  to  the  absorbers  where  the  energy  is  the scattering of the di users. Significant di erences are found between the vertically and horizontally reduced directing , expdi lain users, ing wh wher y bo e the th vertical horizont a ally ected oriented C and Tand to vert a gric eater ally  extent. oriented The di vertically ffusers ha oriented ve similar  50 20 di users reduced the sound waves in the horizontal plane and directed the sound into the ceiling, values in terms of sound strength. A reason for the effect on speech clarity and reverberation time is  showing the importance of directional scattering in rooms with ceiling treatment, which correlates the  scattering  of  the  diffusers.  Significant  differences  are  found  between  the  vertically  and  well to the di usion characteristics of the di using elements. horizontally  directing  diffusers,  where  the  vertical  affected  C50  and  T20  to  a  greater  extent.  The  In the experiments using di users in the ceiling, important improvements for receivers located vertically oriented diffusers reduced the sound waves in the horizontal plane and directed the sound  outside the direct sound field could be achieved. Keeping the sound energy level, a significant increase into the ceiling, showing the importance of directional scattering in rooms with ceiling treatment,  in speech clarity was found for the receiver at the back. This is an important application for use in which correlates well to the diffusion characteristics of the diffusing elements.  classrooms where a teacher gives instructions at the front of the room. An interesting finding with In the experiments using diffusers in the ceiling, important improvements for receivers located  outside  the  direct  sound  field  could  be  achieved.  Keeping  the  sound  energy  level,  a  significant  increase in speech clarity was found for the receiver at the back. This is an important application for  use in classrooms where a teacher gives instructions at the front of the room. An interesting finding  with all the ceiling diffuser configurations was that a decrease in T20 was obtained while G increased,  and this finding also applied when evaluating the results on an average basis for the twelve different  measurements.  This  raises,  again,  the  question  of  the  importance  of  considering  the  effect  of  scattering.  Acoustics 2020, 2 520 all the ceiling di user configurations was that a decrease in T was obtained while G increased, and this finding also applied when evaluating the results on an average basis for the twelve di erent measurements. This raises, again, the question of the importance of considering the e ect of scattering. It should be observed that C only gives information about the early-to-late ratio of the reflections, and does not explain anything about the sound energy. This means that high C can be achieved without guaranteeing sound energy will be suciently high for the listener, or supporting the speaker. A case with high C and low sound energy can result in: (1) Too low a sound level reaching listeners in the rear area of the classroom; (2) Greater voice e ort for the speaker. In the configurations with di users, an increase in C could retain the sound strength, i.e., the sound energy. In choosing the acoustic treatment, it is important to consider the type of room acoustic properties required for the specific room. With di users, reverberation time can be lowered and the ratio of early reflections increase with sound energy conserved. Thus, for environments where complex tasks requiring concentration are performed or in a very noisy environment, sound energy reducing treatment should be used. It is thus of importance to define the activity taking place in the room when choosing the acoustic treatment. In the discussion of acoustic design, it is important to note that the di users used in this study were designed to operate as scattering objects for the higher frequency range, which is important for speech, and for absorption at low frequencies. It is possible to design the di users to operate at the requested frequencies. It has been mentioned above that only small e ects were seen for some configurations. It should be noted that only a small part of the wall area was covered, for example, the ceiling di users covered 8% of the ceiling area. An increased area of acoustic treatment would, to a certain degree, a ect the values further. However, realistic conditions must be considered, e.g., a real classroom can have more furniture a ecting the scattering properties. Other factors that can influence the perception of the acoustics, not studied in this investigation, are background noise, the people and their activities. An observation from the results is the importance of using several room acoustic parameters to characterize a specific acoustic environment (see e.g., Figure 15). For example, T and C can be 20 50 varied with di users, but still maintain the sound strength. However, using absorbers, the sound strength can be varied as well. The outcome of these two scenarios will be di erent experienced, and needs to be considered to obtain the correct acoustic balance for low and high frequencies in ordinary public rooms. 5. Conclusions A clear trend in how di erent types of acoustic treatment a ect room acoustic parameters has been demonstrated where, as a baseline, an acoustic ceiling should be used to decrease the energy level, increase the ratio of early-to-late reflections and lower the reverberation time. However, in a sparsely furnished room, it can still be dicult to achieve a high-quality sound environment with only an absorbent ceiling. The room acoustic parameters can be fine-tuned by using di users and absorbers on the walls and/or in the ceiling. The two di erent types of treatment operate di erently and create distinctive experiences for the people in the room. With additional porous absorbers, G and T are decreased and C is increased, while di users a ect C and T , keeping G stable or increased, 20 50 50 20 depending on placement and amount. The di users used in this study where ecient in absorbing sound at the frequency of 125–250 Hz, but the frequency range within which it operates as a resonant absorber can easily be adopted. Additionally, the frequency ranges for which it operated most eciently as a scattering object can be adjusted, depending on the demands. Acoustics 2020, 2 521 This study provides information on how di erent acoustic treatments can be used to obtain di erent room acoustic qualities, and can be used to improve the sound environment in ordinary public rooms. However, the target values of the room acoustic parameters must be defined for the specific environment and activity, in order to use this information for correct the fine-tuning of acoustic environments. 6. Patents Patent pending, European Patent Oce, 20170667.8. Author Contributions: Writing—original draft preparation: E.A.; Writing—significant contribution to introduction paragraph: E.N.; Experiments—design and accomplishment: E.A. and E.N.; Experiments—analysis: E.A.; Writing—editing and review: E.A.; Structure of paper and supervision: E.N., O.J.I.K. and D.B.H. All authors read and agreed to the published version of the manuscript. Funding: This research is funded by Saint-Gobain Ecophon. Acknowledgments: The authors are grateful for the support from Ecophon providing material, laboratory equipment and laboratory facilities making it possible to perform the experiments. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations Acoustic Configurations CA Ceiling absorptive CD Ceiling di user F Furniture, the room is sparsely furnished WA Wall absorbers VWD Vertical wall di users HWD Horizontal wall di users LFMA Low frequency mineral wool absorber Appendix A Table A1. 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Journal

AcousticsMultidisciplinary Digital Publishing Institute

Published: Jul 4, 2020

Keywords: room acoustics; sound diffusion; sound absorption; sound scattering; sound strength; speech clarity; reverberation time

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