In recent years, the study of natural fibres has been increasing rapidly due to their potential in a wide range of applications. Natural fibres are advantageous in several aspects. Some of the key advantages include affordability, availability, biodegradable, environmentally friendly, lightweight, and good mechanical properties [1
]. Unsurprisingly, there are also disadvantages of natural fibres concerning issues such as durability and fire resistivity. However, researchers are already exploring means to improve these drawbacks as highlighted by a recent review paper [3
]. As the community is becoming more aware of the importance of environmental impact, natural fibres may be a potential alternative to synthetic fibres in certain applications. One of these applications include low-frequency noise reduction.
Consider a typical airport, noise generated within the environment is dominantly contributed by aircraft engines. Generally, the noise generated by the engines is of a low-frequency (<500 Hz) and transient nature due to the variation in the rotating speed of the compressor blades and fan blades during the take-off or landing of the aircraft. This noise can be excessively loud to the extent of adversely affecting the auditory system of the ground crew. Excessive exposure to aircraft noise has also been reported to contribute partly to psychological and physiological health issues [9
]. Some of these health issues are critical such as cardiovascular disease and hypertension. Additionally, the ground crew may experience an increased stress level while performing their routine tasks. Certainly, it is a priority to minimise such health issues that may be experienced by the ground crew.
Clinical studies have been performed to gain better understanding of the underlying biochemical and molecular mechanisms of hair cell death in the cochlea [10
]. The death of hair cells is an irreversible process, resulting in hearing loss of the human. To date, clinical research is ongoing to understand noise-induced hearing loss. For example, Ralli et al. [12
] found that workers—limited to only Italians—were likely to experience noise-induced hearing loss if they were exposed to a noisy working environment over an extended period of employment. The findings were consistent with a recent study by Masterson et al. [13
], which instead focused on the Americans. Rosler [14
] highlighted that a larger population with hearing loss would be expected in developing countries because of lesser emphasis on hearing protection programs.
In practice, hearing protection devices (HPDs) are provided to the ground crew to reduce noise exposure throughout their working hours. The common types of HPDs are earplugs and earmuffs with each having their unique advantages and disadvantages. Earplugs are advantageous for being disposable, inexpensive, and easy to use. Unfortunately, improper fitting of the earplugs compromises the intended performance in noise attenuation [15
]. An obvious solution is to provide workers with proper training, albeit shown to be ineffective by Toivonen et al. [16
]. They reported that poorly fitted earplugs may lower the intended performance in noise attenuation by up to 10 dB. Expectedly, if the earplugs are poorly fitted, leakage paths may exist and permit exterior noise to enter the ear canals directly instead of first propagating through the earplugs and then into the ear canals. Nonetheless, Toivonen et al. [16
] emphasised that incompatible ear canals may also contribute to the drop in acoustical performance of the earplugs.
Considering the characteristic of aircraft noise, earmuffs are typically recommended over earplugs for better hearing protection [17
]. This recommendation is because typical earmuffs can achieve a noise attenuation of about 9 dB per octave between 125 Hz and 1 kHz before reaching a saturation point of around 35 dB noise attenuation above 2 kHz. Earplugs, on the other hand, are typically more effective for noise attenuation above 1 kHz with the assumption that a proper fit is ensured by the user [18
Earmuffs may be costly and bulky but their acoustical performance is independent of the ear canals and require minimal user knowledge. As compared to the earplugs, the user is less likely to wear the earmuff incorrectly. Hence, the intended acoustical performance of a given earmuff may likely be achieved in contrast to the earplugs. Nonetheless, a drop in acoustical performance of the earmuffs can be expected in the field as compared to that achieved from a laboratory test because the earmuffs are usually worn in an ideal manner without much considerations of comfort [18
]. For example, a tight headband may provide a better seal around the pinnae but the forces exerted on the temporal bones may lead to discomfort. Additionally, personal protective equipment, such as eye goggles, face shields and helmets, and facial hair may also adversely influence the acoustical performance of the earmuffs.
An extensive range of earmuffs is available commercially for general use, including proprietary models designed for specific noise environments. Expectedly, the proprietary models come with a hefty price tag and may also be much heavier. Alternatively, the acoustical performance of typical earmuffs can be improved by modifying the assembly components—namely the cushion pads, headband, ear cups, and inner foam lining [19
] recently showed that natural fibre-reinforced polymer composite earmuffs could improve the noise reduction at certain frequencies. The acoustical performances of several synthetic fibre-reinforced polymer earmuffs were measured and compared with various commercial earmuffs. The acoustical performances of the composite ear cups were found to be different depending on the noise environment—continuous and impulse—albeit not discussed in detail. Apart from this study, limited reports with similar interest can be found in the literature.
Earlier studies focused on the viability of natural fibres rather than the understanding of their acoustical properties [7
]. Recent studies, however, showed a rise in research effort to understand the acoustical properties of natural fibres. For example, coir fibre was considered in place of synthetic fibre as an absorptive layer behind a micro-perforated panel [24
]. Results showed remarkable sound absorption properties (up to 0.95) of the panel assembly above 1 kHz. Unfortunately, the findings were unable to represent the acoustical properties of coir fibre independently. This gap was later addressed in a separate study by the same research group [26
]. The follow-up study investigated several parameters and found that, if properly designed, coir fibre could achieve good sound absorption properties below 1 kHz. At that point in time, the conclusion was made based on numerical simulations without any validation work, until much later [27
]. To date, research efforts are still ongoing to understand and improve the acoustical performance of coir fibres [28
Coir fibres have been characterised based on sound absorption properties but their transmission behaviour has yet to be investigated, especially in the context of industrial applications. This research gap motivated the present work to consider coir fibre-reinforced polypropylene (Coir-PP) as an alternative to the material selection for the ear cups of earmuffs. Additionally, a hybrid model—coir/carbon fibre-reinforced polypropylene (Coir/C-PP)—was also considered. This work aims to demonstrate the feasibility of composite earmuffs for low-frequency noise reduction in continuous and transient noise environments. The former and the latter were emulated by pink noise and aircraft take-off exterior noise, respectively. The acoustical performances of the composite earmuffs were evaluated experimentally in a reverberation chamber. Consequently, the proposed earmuffs may find potential applications in noise environments that are predominantly low-frequency—at airports, at construction sites, and in some automobile cabins, for example [30
]. In Section 2
, the specimen details and experimental approach are elaborated. In Section 3
, the acoustical performances of the composite and the commercial earmuffs are presented and discussed further in Section 4
. Section 5
provides conclusions based on the earlier sections.
Coir-PP and Coir/C-PP were considered as potential alternatives to the material selection of the ear cups of typical earmuffs. To reiterate, this work aims to explore the viability of composite earmuffs in low-frequency noise reduction in contrast to a reference earmuff. Results indicated a difference in IL of both composite earmuffs in continuous noise environment (pink noise) and transient noise environment (aircraft take-off exterior noise).
In continuous noise source, Coir-PP earmuffs showed improvements in IL in contrast to the reference earmuff at 320–512 Hz. This improvement, however, resulted in a compromise in IL below 192 Hz albeit this compromise was found to reduce with the inclusion of carbon fibre into the Coir-PP ear cups. In the higher frequencies (>1120 Hz), Coir-PP earmuff achieved up to 8.2 dB improvement in IL. This improvement increased further by 3.8 dB when the hybrid ear cups (Coir/C-PP) were considered, suggesting that the loudness perception could be reduced by more than half.
Interestingly, otherwise was observed in transient noise environment where a higher IL was observed in the lower frequencies (160–544 Hz) for both composite earmuffs. In contrast to the reference earmuff, the improvement in IL went up to 8.6 dB at 256 Hz. Although both composite earmuffs showed marginal drop in IL in the higher frequencies (>1952 Hz), it was not crucial because low-frequency noise reduction is the key interest. The improvements in IL achieved by the composite earmuffs could be attributed to the high damping property of the natural fibres. Such property is usually low in thermoplastics—ABS, for example.
As highlighted in Section 2.1
, the assembly components—headband, inner foam lining, and cushion pads—of the earmuffs and the physical profile of the ear cups were kept identical. As such, it would be interesting to discuss the cost to produce the respective ear cups. Based on Table 1
, the estimated cost to produce a pair of the reference, the Coir-PP, and the Coir/Carbon-PP ear cups would be $0.39, $0.17, and $0.45, respectively (Singapore dollar). The estimated cost suggests that the composite earmuffs may provide a cheaper and greener alternative in reducing low-frequency noise as opposed to the proprietary earmuffs. It should be noted that the cost of the reference ear cups was calculated based on only the material cost without taking into consideration of the manufacturer’s branding, which could further increase the cost. Moreover, the reduced weight (up to 29%) of the composite earmuffs in relative to the reference earmuff suggests that the user may be more likely to wear the earmuff over an extended duration of time.
Future work may include the numerical simulations to gain better understanding of the acoustical performance of the respective ear cups and optimise their designs, resulting in enhanced acoustical performance. Additionally, a modal analysis may also be considered to understand the acoustic-structure interaction between the ear cups and the surrounding components, such as the cushion pad, inner foam lining, pinna, and the enclosed fluid cavity. Different manufacturing parameters may also be considered to understand their influence on the acoustical performance of the composite ear cups—processing temperature and pressure, for example. Various types of transient noise sources may also be considered to evaluate the composite earmuffs and understand their potential in such noise environments. Additionally, the present study may also be extended for clinical studies with the possibility to analyse temporary threshold shift and distortion product otoacoustic emissions on human subjects.