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Hindawi Advances in Acoustics and Vibration Volume 2018, Article ID 2057820, 10 pages https://doi.org/10.1155/2018/2057820 Research Article Low-Frequency Noise Reduction by Earmuffs with Flax Fibre-Reinforced Polypropylene Ear Cups 1,2 3 3 Linus Yinn Leng Ang , Le Quan Ngoc Tran, Steve Phillips, 2 1 Yong Khiang Koh, and Heow Pueh Lee Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575 Kinetics Design and Manufacturing, Singapore Technologies Kinetics Ltd., 249 Jalan Boon Lay, Singapore 619523 Singapore Institute of Manufacturing Technology (SIMTech), Agency for Science, Technology and Research (A*STAR), 73 Nanyang Drive, Singapore 637662 Correspondence should be addressed to Linus Yinn Leng Ang; firstname.lastname@example.org and Heow Pueh Lee; email@example.com Received 19 September 2017; Revised 24 November 2017; Accepted 28 November 2017; Published 3 January 2018 Academic Editor: Kim M. Liew Copyright © 2018 Linus Yinn Leng Ang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Soldiers and supporting engineers are frequently exposed to high low-frequency (<500 Hz) cabin noise in military vehicles. Despite the use of commercial hearing protection devices, the risk of auditory damage is still imminent because the devices may not be optimally customised for such applications. This study considers flax b fi re-reinforced polypropylene (Flax-PP) as an alternative to the material selection for the ear cups of commercial earmuffs, which are typically made of acrylonitrile butadiene styrene (ABS). Different weaving configurations (woven and nonwoven) and various noise environments (pink noise, cabin booming noise, and firing noise) were considered to investigate the feasibility of the proposed composite earmuffs for low-frequency noise reduction. The remaining assembly components of the earmuff were kept consistent with those of a commercial earmuff, which served as a benchmark for results comparison. In contrast to the commercial earmuff, the composite earmuffs were shown to be better in mitigating low-frequency noise by up to 16.6 dB, while compromising midfrequency acoustical performance. Consequently, the proposed composite earmuffs may be an alternative for low-frequency noise reduction in vehicle cabins, at airports, and at construction sites involving heavy machineries. 1. Introduction Furthermore, they are inexpensive and, in a way, dispensable. However, poor fitting of earplugs is still a problem observed Soldiers and supporting engineers are commonly exposed among some employees [2, 3], consequently compromising to high cabin noise in military vehicles, which includes noiseattenuation.InastudybyToivonenetal. , they rfi ing noise and/or cabin booming noise. eTh energy content reported a drop in noise attenuation of up to 10 dB when in such noises generally falls in the low-frequency range the earplugs were poorly tfi ted into the ear canals. How- (<500 Hz). Typically, automobile manufacturers apply several ever, the drop might not be necessarily due to the lack passive and/or active acoustical treatments for cabin noise of user knowledge but could also be due to incompatible control. However, the maximum acoustical performance of ear canals. Nonetheless, earplugs are usually inadequate in the treatments may not be achieved due to other design typical military environments. As such, earmuffs are typically considerations . In practice, the simplest approach is to recommended. One key advantage is the nondependency highlight the importance of hearing protection to the soldiers of acoustical performance on one’s ear canals. If properly and engineers on the correct procedures in wearing hearing worn, earmuffs can provide better noise attenuation than protection devices, earplugs and earmuffs (or ear defenders). earplugs . Commercially, a wide selection of earmuffs is Earplugs are generally preferred as they are easy to available, catering for use in different noise environments. put on and do not cause as much discomfort as earmuffs. Some models are even proprietary to gain an advantage 2 Advances in Acoustics and Vibration among the competitors. However, a risk of auditory damage These studies served as the motivation of this work to is still imminent because commercial earmuffs may not consider flax b fi re-reinforced polypropylene (Flax-PP) as be optimised for reducing cabin noise in military vehicles. an alternative to the material selection for the ear cups of Alternatively, the acoustical performance of earmuffs can also earmuffs. Additionally, different configurations were con- be improved by modifying several design parameters such as sidered (woven and nonwoven). eTh present study aims to ear cups, cushions, inner foam lining, and headbands [5–8]. understand the feasibility of composite earmuffs for cabin Recently, Augustine  proposed the use of synthetic noise control in military vehicles where the crew’s exposure fibre-reinforced polymer composite as the material of the to low-frequency noise is still a concern to date . Conse- ear cups. The physical design was based on existing com- quently, the proposed composite earmuffs may be a potential mercial earmuffs used by the crew in military aircraft. In alternative for low-frequency noise control applicable in total, three types of b fi re were considered (aramid, glass, vehicle cabins, at airports, and at construction sites involving andcarbon).Forimpulsenoise,aramidfibrewasshown heavy machineries. eTh next section elaborates the details to yield the highest noise attenuation (up to 28 dB). The onthematerials andmethods.InSection 3, theresultsare other two b fi res fared poorer with noise attenuation of up presented and discussed. In Section 4, the limitations of this to 20 dB, comparable to the existing earmuffs. In contrary, study are discussed and avenues for future work are high- these findings appeared otherwise in a continuous noise lighted. In Section 5, a conclusion is provided based on the environment where the existing earmuffs were shown to findings. be superior. Despite efforts to maintain consistency in the study, some parameters remained inconsistent such as cush- 2. Materials and Methods ions and headbands. Furthermore, the drilled holes at the sides of each ear cup could have introduced sound leakage. This section first presents the details of the commercial Separately, Ahmadi et al.  considered differently by com- earmuff used for benchmarking and the different types of flax fabric considered for the fabrication of the composite ear bining nanoclay with acrylonitrile butadiene styrene (ABS), a common thermoplastic material used for the ear cups cups. Subsequently, the fabrication process of the composite of commercial earmuffs. Experimentally, its acoustical per- earcupsispresented.Thissectionendsoffwithhowthe experiment was performed to obtain the acoustical perfor- formance was compared against single- and double-cup commercial earmuffs. Remarkably, the proposed material mance of the commercial and the composite earmuffs. achieved notable noise attenuation (up to 9 dB) between 250 Hz and 8 kHz as compared to the commercial single-cup 2.1. Specimen Details and Materials. The composite ear cups earmuff. However, such acoustical performance in the low- were geometrically designed with high resemblance to the frequencyrange is stillnot idealfor cabinnoise controlin ear cups of a commercial earmuff (3 M Peltor Optime military vehicles. I H510F). The commercial earmuff served as the benchmark Besides synthetic b fi res, natural fibres—such as flax, against the composite earmuffs. From the benchmarking, hemp, and jute—have also caught research attention due to the acoustical benefits and drawbacks of the composite several advantages. Some of these advantages include good earmuffs would then be clearly identified. Hereinaer ft , for mechanical properties [11–14], good damping properties [12, clarity and brevity, the commercial earmuff is referred to as 13], lightweight [13, 15, 16], low cost [14, 16], biodegradable, the reference earmuff. Apart from the ear cups, the remain- and environmental-friendly [13, 14, 17]. From literature, it ing components—inner foam lining, cushion, and head- is prominent that earlier studies focused on the viability of band—were kept identical to the reference earmuff. In other natural b fi res but not on the understanding of their acoustical words,thecomposite earmuff shared thesameassembly properties. It is only recently when Yang and Li  investi- components as the reference earmuff except for the ear cups. gated the sound absorption properties of several natural b fi res Three different types of flax fabric were considered in by means of an impedance tube. In comparison to synthetic fabricating the composite ear cups. eTh flax fabrics included a fibres, they showed that natural b fi res could exhibit good nonwoven mat, a 2 × 2twill weavemat,and a4×4 sound absorption properties with flax fibre being the highest hopsack weave mat. The nonwoven mat was provided by in terms of noise reduction coefficient (0.65), nearly two times Eco-Technilin (Valliquerville, France), while the remaining higher than glass b fi re. Later, Prabhakaran et al.  extended mats were supplied by Composites Evolution (Derbyshire, the work by demonstrating the damping characteristics of England). For the polymer matrix, polypropylene films were flax b fi re-reinforced epoxy pertaining to sound and vibration. used.ThesefilmsweresuppliedbyThePolyolenfi Company In comparison with glass bre fi -reinforced epoxy, the sound (Singapore) in their unmodified grade (Cosmoplene Y101E). absorption coefficient was shown to be higher at 100 Hz (up Table 1 shows the density of the respective flax fabrics and the to 21%) and beyond 2 kHz (up to 25%), respectively. In terms polypropylene film. of vibration damping, flax b fi re-reinforced epoxy was shown eTh composite ear cups were manufactured using com- to be superior (50% higher) as compared to glass bre fi - pression moulding technique in which the flax fabrics and reinforced epoxy. At this point, these studies established a polypropylene films were stacked in a prescribed sequence consistency in the acoustical properties of flax b fi re. Subse- and placed between a two-piece aluminium mould as shown quently, Mamtaz et al.  highlighted the potential of natural in Figures 1(a) and 1(b). The mould was designed to replicate fibres for noise control applications with a review of related the geometrical profile of the ear cups from the reference studies albeit extensive further work is still necessary. earmuff. Next, the setup was placed into the Collin hot press Advances in Acoustics and Vibration 3 (a) (b) (c) (d) Figure 1: Manufacturing process of the composite ear cups: (a) a stack of flax fabrics and polypropylene films placed on top of one side of the two-piece aluminium mould; (b) hot press compression moulding; (c) unprocessed composite ear cup; (d) processed composite ear cup (bottom image) and assembled earmuff for experiment (top image). Table 1: Density of the respective flax fabrics and the polypropylene Table 2: Fibre volume fraction and number of b fi re layers in each film. composite ear cup. Fabric/film Fabric density [g/m ] Fibre volume Fabric Number of b fi re layers fraction [%] Nonwoven mat 300 Nonwoven mat 4 27 2×2 twill weave mat 400 2×2 twill weave mat 3 27 4×4 hopsack weave mat 500 4×4 hopsack weave Polypropylene film 90 mat (Figure 1(b)) for fabrication. eTh process lasted for 15 minutes 3663), the simulator was placed on a standard test table at the with the processing temperature and the pressure loading centre of the reverberation room with a volume of 226.9 m . maintained at 190 Cand 20bar, respectively.Subsequently, An omnidirectional loudspeaker (Larson Davis BAS001) was thesetupwascooledtoroomtemperatureat arateof−10 C placed at one of the room corners to transmit pink noise per minute. The fabricated composite ear cup (Figure 1(c)) (50–12,000 Hz), which was generated and amplified by a was then prepared by removing the asset materials and noise generator (Bruel ¨ & Kjær Type 1405) and a signal assembling them with the remaining components—inner amplifier (Larson Davis BAS002), respectively. An additional foam lining, cushion, and headband—to form the earmuff as microphone was positioned at 1 m away from each side of showninFigure1(d). Notethat thefabrication processcould the simulator’s ear to record the sound pressure level (SPL) only produce one composite ear cup at a time. er Th efore, during each measurement. The additional measurements the fabrication process was repeated to obtain the required served to ensure consistency in the reverberant sound efi ld number of composite ear cups. Figure 2 provides a closed-up as that recorded by the simulator without any earmuff (open view of each composite ear cup assembled with the remaining ear). As for subsequent stages of the experiment involving components of the earmuff except the headband. Table 2 rfi ing noise and cabin booming noise, the omnidirectional presents the b fi re volume fraction and the number of layers loudspeaker was substituted by a pair of active loudspeakers in each composite ear cup. (Yamaha DXR15) due to technical limitations. Similarly, the loudspeakers were positioned at two of the room corners to 2.2. Experimental Details. The experiment adhered to the transmit the respective audio signals—downloaded from the guidelines in BS EN ISO 4869-3  except for the use of a Internet—which were played via an audio system (Sony ZS- sound quality head and torso simulator (Bruel ¨ & Kjær RS70BT). Type 4100) as opposed to a cylindrical acoustic test fixture. It must be emphasised that the simulator was designed Together with a data acquisition unit (Bruel ¨ & Kjær Type for evaluating sound quality in automobile cabins and other 4 Advances in Acoustics and Vibration (a) (b) (c) Figure 2: Closed-up view of the composite ear cups assembled with the inner foam lining and cushion: (a) nonwoven; (b) 2×2 twill weave; (c)4×4 hopsack weave. optimisation studies . Therefore, the presented results measurements were then postprocessed and computed in would only serve as relative measurements for benchmarking terms of insertion loss (IL) as defined by the SPL difference of the composite earmuffs with the reference earmu.ff eTh between with and without the earmuff worn on the simulator, same limitation was highlighted in a recent study by Ang et which is given by [25, 26] al. . In this case, the presented results must not be IL =𝐿 −𝐿 , (1) 𝑓 wo,𝑓 w,𝑓 misunderstood as a direct indication of real-ear attenuation values, which is one of the most accurate evaluation methods where the subscript𝑓 denotes a frequency-dependent term for hearing protection devices . and𝐿 and𝐿 denotethe time-averagedSPL withoutand wo w Despite the simulator’s design to emulate the head and with the earmuff worn on the simulator, respectively. Based torso of a human as close as possible, leakage paths would still on the assumption of a diffuse efi ld, both terms ( 𝐿 and𝐿 ) wo w exist differently in both cases. Having this concern in mind, were taken as the average between the SPL at both ears of the efforts were made to minimise experimental uncertainty, simulator. Figure 3(b) shows an overview of the experimental which could be caused by possible leakage paths that may setup in the reverberation room. exist in the simulator. Prior to the donning of each earmuff on the simulator, a visual inspection was performed to make 3. Results sure that the moulded pinnae remained well-tfi ted in their This section presents and discusses the acoustical perfor- allocated recesses. This inspection was necessary due to the mance of the earmuffs in the respective noise sources. The likelihood that the process of removing the earmuff from results were plotted in narrowband frequency range rather thesimulator couldaeff ct thefittingofthe mouldedpinnae. than octave band frequency range, which should be the case Experimental uncertainty was further minimised by main- in the context of evaluating hearing protection devices . taining consistency in the earmuff’s position and tfi ting on The intention was to better illustrate the overall charac- the simulator. This consistency was achieved by noting the teristics of the composite earmuffs, which are crucial for scales around thepinnaeofthesimulatorandon topofits future developmental studies to improve their acoustical per- head, which could be partially seen in Figure 3(a). Lastly, formance.TheILcurvesare boundedbyashaded region, discrepancy in headband force between each earmuff was representing the upper and the lower limits of the expanded minimised by maintaining the extended length of the head- uncertainty at a confidence level of 95% (i.e., coverage factor band. Note that, to measure the headband force, a specially = 2) . Systematic error was taken to be 0.5 dB. designedtestrig wouldberequiredasshownbyHsuetal. , for example. Such consideration would require expertise and was, therefore, beyond the scope of the present study. 3.1. Acoustical Performance of the Composite Earmuffs in Pink Six measurements were recorded for each earmuff in each Noise. The IL curve of each earmuff was first determined in type of noise source. This number of measurements exceeded pink noise. Figure 4(a) shows the typical spectrum of the pink the recommendation of the test standard (at least three noise generated in the reverberation room. To identify the measurements). The purpose of exceeding the standard’s rec- acoustical benefits and drawbacks of the composite earmuffs, ommendation was to reduce experimental uncertainty from each IL curve was compared against that of the reference the average of more datasets. eTh measurement duration was earmuff (Figures 4(b)–4(d)). However, a comparative study 30 s for pink noise and cabin booming noise, while the to determine the ideal composite earmuff was not presented measurement duration was 10 s for firing noise. Additionally, due to differences in their composition. the SPL of each noise source was ensured to be at least In Figures 5(b)–5(d), the overall trend of the IL curves for 15 dB higher than the background noise in the room. eTh each composite earmuff was the same despite the differences Advances in Acoustics and Vibration 5 Reference Earmuff Loudspeaker Microphone Simulator Scales Composite Earmuff (a) (b) Figure 3: (a) Closed-up view of the simulator with the composite earmuff worn based on the scales; (b) overview of the experimental setup in the reverberation room with the reference earmuff worn on the simulator. 120 50 40 −10 100 1000 10000 100 1000 10000 Frequency (Hz) Frequency (Hz) Open Ear Reference Nonwoven (a) (b) 50 50 30 30 10 10 −10 −10 100 1000 10000 100 1000 10000 Frequency (Hz) Frequency (Hz) Reference Reference 2 × 2 4 × 4 (c) (d) Figure 4: (a) Averaged sound pressure level of the pink noise in the reverberation room measured by the simulator without any earmuff (open ear); comparison between the insertion loss curve of (b) the reference and the nonwoven composite earmuffs; (c) the reference and the 2×2 twill weave composite earmuffs; and (d) the reference and the 4×4 hopsack weave composite earmuffs. Shaded area is bounded by the upper and the lower limits of the expanded uncertainty at a confidence level of 95%. IL (dB) SPL (dB) IL (dB) IL (dB) 6 Advances in Acoustics and Vibration 120 50 −10 50 500 5000 50 500 5000 Frequency (Hz) Frequency (Hz) Open Ear Reference Nonwoven (a) (b) 50 50 30 30 10 10 −10 −10 50 500 5000 50 500 5000 Frequency (Hz) Frequency (Hz) Reference Reference 2 × 2 4 × 4 (c) (d) Figure 5: (a) Averaged sound pressure level of the cabin booming noise in the reverberation room measured by the simulator without any earmuff (open ear); comparison between the insertion loss curve of (b) the reference and the nonwoven composite earmuffs; (c) the reference and the2×2 twill weave composite earmuffs; and (d) the reference and the 4×4 hopsack weave composite earmuffs. Shaded area is bounded by the upper and the lower limits of the expanded uncertainty at a confidence level of 95%. in their composition. Between 100 and 200 Hz, an IL dip was oftheaircavitywouldresultinahigh-frequency acoustic res- consistently observed across the different earmuffs. Knowing onance. Referring to Figures 4(b)–4(d), the IL dip at around that the physical geometries of the composite ear cups were 4.5–5.1 kHz could be attributed to the cavity resonance. This kept identicaltothatofthereferenceearmu,t ff hecause ofthe deduction could be drawn from the understanding that the IL dip would not be attributed to the assembly components of volume ofthecavityineachtypeofearcupwouldbenearly the earmuff. This understanding narrowed the list of possible identical since their physical geometries were maintained causes of the IL dip to either the acoustic resonance of the consistent. Intuitively, the cavity resonance would occur enclosed air cavity or the pumping motion of the ear cups. at around the same frequency, which could be at around The former could be explained by considering the math- 4.5–5.1 kHz as mentioned earlier. This understanding could ematical formulation to approximate acoustic resonance in be further reinforced by Boyer et al.  where they found an enclosed volume. Evidently, the air cavity enclosed by the that cavity resonance generally occurs at high frequency, ear cup would take the boundary profile of a highly irregular consistent across the tested earmuffs. In their case, the cavity shape. However, Boyer et al.  and Paurobally and Pan  resonance occurred at around 4.0–4.5 kHz. mentioned that the acoustic stiffness of the air cavity could be Based on the above understanding, it is highly possible approximated by considering it as a hemicylindrical volume. that the IL dip at around 100–200 Hz was attributed to the In this case, the acoustic stiffness of the air cavity 𝑘 is given pumping motion of the ear cups, which is dependent on the air by  design parameters and the leakages around the cushion pads . Boyer et al.  revealed that the pumping motion of the 2 2 ear cups would highly influence the acoustical performance 𝜌𝑐 𝑆 (2) 𝑘 = , air of the earmuff at low frequency. Separately, Boyer et al.  mentioned that the pumping motion of the ear cups would where𝜌 and𝑐 denote the density of and the sound speed typically occur at around 100–300 Hz. Hence, it could be inferred that the IL dip at around 100–200 Hz was attributed in air, respectively, in the ear cup; 𝑆 denotes the cross- sectional area of the cavity; and𝑉 denotes the volume of to the pumping motion of the ear cups. As mentioned earlier, the cavity. Knowing that the acoustic resonance of the air the pumping motion of the ear cups is not only dependent cavity is inversely proportional to the square root of the on the design parameters, but also the leakages around acoustic stiffness, it could be deduced that the small volume the cushion pads. Typically, it is challenging to achieve a IL (dB) SPL (dB) IL (dB) IL (dB) Advances in Acoustics and Vibration 7 perfect seal due to a nonuniform curvature profile around the earmuffs. Conversely, the nonwoven earmuff was marginally ears. To minimise such leakages, the headband force could be better by 1.5 dB. increased, applying more pressure on the temporal bone. For At high frequencies (1985–2425 Hz), better IL of up to instance, Boyer et al.  demonstrated that a higher head- 6.8 dB was achieved albeit not as prominent for the nonwoven band force would achieve a better sealing of the cushion pads, earmu.ff Beyond this point, no noticeable dieff rences in leading to an improved low-frequency IL of the earmuffs. acoustical performance were observed for the composite However, the resulting high pressure would compromise earmuffsincontrasttothereferenceearmu.ff Nonetheless, comfort and might discourage the user from wearing the the significance of the acoustical performance exhibited by earmuff over an extended period of time . the composite earmuffs in the mid- and high frequencies may not be crucial in low-frequency noise control applications. In contrast to the reference earmuff, a higher IL was achieved in the low-frequency range (<450 Hz) by all com- 3.3. Acoustical Performance of the Composite Earmuffs in posite earmuffs. This observation was more prominent for Firing Noise. eTh frequency content of rfi ing noise in vehicle the4×4 hopsack weave and the nonwoven earmuffs with cabins is similar to cabin booming noise where most of the an IL of up to 5.6 dB at 256 and 320 Hz, respectively. eTh noise energy falls below 500 Hz as illustrated in Figure 6(a). 2×2 twill weave earmuff fared poorer with an IL of only up Figures 6(b)–6(d) show the comparison of the IL curves to 3.6 dB at 320 Hz. Thereaer ft , the IL of all composite between the reference earmuff and the respective composite earmuffs gradually decreased as the frequency approached earmuffs. the second IL dip at 640 Hz. This dip caused a drop in IL for the composite earmuffs between 510 and 1090 Hz. Conversely, The overall trend of the IL curves was again similarly the same observation was not made for the reference earmuff. observed for the composite earmuffs including the second IL dip. However, the range of improved acoustical performance Basedonthesimilarity inoccurrence oftheILdipobserved for each composite earmuff, the second IL dip could be due to in the low-frequencies was observed to be slightly narrower (160–360 Hz) as opposed to the earlier n fi dings in Sections one of the structural resonances of the composite ear 3.1 and 3.2. Nonetheless, decent IL of 9.8–10.3 dB was still cups. achieved, particularly at 208 Hz, by the composite earmuffs. At the high-frequency range, more IL dips were observed The second IL dip was observed to be less distinct with for the composite earmuffs at 2560, 4480, and 7040 Hz. a wider bandwidth ranging from 560 to 720 Hz. Despite this Keeping in mind the intention of this study, these dips are dip,thecompromiseinacousticalperformanceinthemidfre- not as critical since conventional acoustical treatments in quencies (400–1008 Hz) remained comparable in bandwidth the vehicle cabins are likely sufficient to complement noise as opposed to the earlier ndin fi gs in Sections 3.1 and 3.2. attenuation at such high frequencies . Nonetheless, with However, a minor downward shift in frequency was observed. such acoustical performance achieved by the composite The 2×2 twill weave and the4×4 hopsack weave earmuffs earmuffs in the low-frequency range, it was of great interest were shown to reach a maximum drop in IL at 608 Hz to evaluate their practicality in vehicle cabins. of 14.3 dB and 12.4 dB, respectively. As for the nonwoven earmu,t ff hemaximumdropinILis11.6dBat736Hz.Beyond 3.2. Acoustical Performance of the Composite Earmuffs in 1008 Hz, no noticeable differences in acoustical performance Cabin Booming Noise. Booming noise is an undesirable were observed for the composite earmuffs in contrast to the phenomenon within a vehicle cabin. Generally, most of the reference earmu.ff noise energy falls below 500 Hz as illustrated in Figure 5(a). Figures 5(b)–5(d) show the comparison of the IL curves 4. Discussion between the reference earmuff and the respective composite earmuffs. The findings of this present study were based on solely A similar global trend in IL was observed for all com- experimental work. Analytical models could be used to posite earmuffs, consistent with the findings from Section 3.1. predict the IL curves of each earmuff as a form of validation. Again, thefirstILdipwasobservedfor allearmuffs.However, Generally, analytical models aredeveloped basedonthe for the composite earmuffs, only the second IL dip (640 Hz) concepts of lumped parameters modelling. Boyer et al. , was prominent. eTh absence of the other IL dips could be due for instance, provided a comprehensive review in this aspect. to the inherent low acoustical energy of the cabin booming In their review, they stated that analytical models would only noise at high frequency (Figure 5(a)). provide an approximation of the acoustical performance for Likewise, better IL in the lower frequencies (128–416 Hz) a given earmuff up to about 1 kHz and before the occurrence was achieved by the composite earmuffs as opposed to the of the rfi st acoustic or structural resonance. Considering reference earmu.ff In this frequency range, an IL of up to typical environmental noises, it is essential to understand the 16.6 dB was achieved by the2×2 twill weave earmu,ff while acoustical performance of the earmuff above 1 kHz. In this the other composite earmuffs fared slightly poorer by 1-2 dB. case, the limitation could be addressed by numerical methods Again, the acoustical performance for all composite earmuffs such as finite element method or boundary element method. beyond 450 Hz decreased progressively towards the second IL Numerical methods are generally suitable to approximate dip at 640 Hz, aeff cting unfavourably up to 1090 Hz. As such, the acoustical performance of earmuffs up to about 5 kHz a compromise in acoustical performance by up to 16.6 dB was  where the limitations now lie on the mesh density observed for the2×2 twill weave and the4×4 hopsack weave of the model and the computational resources required 8 Advances in Acoustics and Vibration −10 50 500 5000 50 500 5000 Frequency (Hz) Frequency (Hz) Reference Open Ear Nonwoven (b) (a) 50 50 −10 −10 50 500 5000 50 500 5000 Frequency (Hz) Frequency (Hz) Reference Reference 2 × 2 4 × 4 (c) (d) Figure 6: (a) Averaged sound pressure level of the firing noise in the reverberation room measured by the simulator without any earmuff (open ear); comparison between the insertion loss curve of (b) the reference and the nonwoven composite earmuffs; (c) the reference and the 2×2 twill weave composite earmuffs; and (d) the reference and the 4×4 hopsack weave composite earmuffs. Shaded area is bounded by the upper and the lower limits of the expanded uncertainty at a confidence level of 95%. for high-frequency prediction. Although numerical methods required could lead to a thesis by itself . Consequently, could address the limitation of analytical models, challenges with a robust and accurate model, parametric study can be remain—to date—where the material properties and the performed to optimise the design of the ear cups for enhanced interaction between each component of the earmuff must be acoustical performance. correctly specified. Else, erroneous results would be expected. This statement could be supported by the literature where 5. Conclusions researchers have attempted to develop a robust numerical modeltoapproximate—withhighaccuracy—thenoise atten- In conclusion, this study proposed the potential of improving uation of a given earmuff [28, 31, 32]. As demonstrated by low-frequency noise reduction of commercial earmuffs by Boyer et al. , rigorous experiments would be necessary considering Flax-PP as an alternative to the material selection to obtain important parameters such as the headband force for the ear cups. The material, however, involved a trade-off in and the cushion dynamic stiffness. Recently, researchers acoustical performance at the midfrequencies (510–1090 Hz) have attempted to increase the complexity of the numerical in contrast to a reference earmuff. Three different types of flax model by considering the influence of the human skin, ear fabrics were considered (nonwoven mat,2×2 twill weave, canal, cartilage, and bone [33, 34]. Evidently, the increase in and4×4 hopsack weave). Due to the differences in their modelling complexity would result in the need for higher composition, a comparative study to determine the ideal computational resources as pointed out by Sgard et al. composite earmuff was not considered. . However, despite the high complexity of the numerical For practicality, the potential of the composite earmuffs model, large discrepancies between the measurement and the was demonstrated by evaluating their acoustical performance simulation were still observed . under two types of cabin noise encountered in military In the present study, the composite ear cups would vehicles (booming noise and firing noise). Results showed inevitably increase the complexity of the numerical model an improvement in IL by up to 16.6 dB within the range of considering that the material of the ear cup is no longer 128–416 Hz in booming noise and up to 10.3 dB within the homogeneous or isotropic in contrast to ABS. As such, the range of 160–360 Hz in firing noise. Future work to optimise present study provides an avenue for future work to develop the composition and physical design of the composite ear the numerical model of the composite ear cups to predict cups is imperative to improve their acoustical performance the respective IL curves, not to mention that the eoff rt and possibly reduce the compromise at the midfrequencies. SPL (dB) IL (dB) IL (dB) IL (dB) Advances in Acoustics and Vibration 9 Consequently, the proposed composite earmuffs may be  F. Duc, P.-E. Bourban, and J.-A. E. Man ˚ son, “Dynamic mechan- ical properties of epoxy/flax b fi re composites,” Journal of Rein- a potential alternative for low-frequency noise reduction forced Plastics and Composites,vol.33, no.17, pp.1625–1633, applicable in vehicle cabins, at airports, and at construction sites involving heavy machineries.  L.Pil,F.Bensadoun,J. Pariset,andI.Verpoest, “Whyare designers fascinated by flax and hemp b fi re composites?” Conflicts of Interest Composites Part A: Applied Science and Manufacturing,vol.83, pp.193–205,2016. eTh authors declare that they have no conflicts of interest.  L. Yan, N. Chouw, and K. Jayaraman, “Flax b fi re and its composites—a review,” Composites Part B: Engineering,vol.56, Acknowledgments pp. 296–317, 2014.  Y. Swolfs, L. Gorbatikh, and I. Verpoest, “Fibre hybridisation The authors gratefully acknowledge the na fi ncial supports in polymer composites: A review,” Composites Part A: Applied provided by Singapore Economic Development Board (EDB) Science and Manufacturing,vol.67,pp.181–200,2014. and Singapore Technologies Kinetics Ltd. (ST Kinetics) under  S. Prabhakaran, V. Krishnaraj, M. Senthil Kumar, and R. the EDB Industrial Postgraduate Program (EDB-IPP) (Grant Zitoune, “Sound and vibration damping properties of flax no. 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