## 4 Discussion  
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.  
Figure 6: Insertion loss curves of the reference earmuff (dark grey), the Coir-PP earmuff (red), and the Coir/C-PP earmuff (blue) in aircraft take-off exterior noise. Shaded area is bounded by the upper and the lower limits of the expanded uncertainty at a confidence level of 95%. Note that the frequency interval is 8 Hz.  
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.