## 3 Results
### Acoustical Performance of the Earmuffs in Pink Noise  
The acoustical performance of each earmuff was first evaluated based on the IL in a continuous noise environment. Figure 4 shows the IL curves of the earmuffs in pink noise. It should be noted that the results were truncated at 6 kHz, as no significant differences were observed above this frequency.  
Based on the mean values, Coir-PP earmuff showed better IL within a narrow bandwidth (320-512 Hz) in contrast to the reference earmuff. Although this improvement came with a compromise in IL below 192 Hz, the inclusion of carbon fibre brought the IL closer to that of the reference earmuff. At higher frequencies (\(>\)1120 Hz), the acoustical performance of Coir-PP earmuff showed improvements in IL by up to 8.2 dB. This value increased by 3.8 dB at the same frequency (4928 Hz) with Coir/C-PP earmuff. Having a total noise reduction of 12 dB, more than half the perceived loudness could be reduced.  
In general, the overall trend of the IL curves was similar with a distinct IL dip at around 170 Hz. Since the physical design of the composite ear cups was kept identical to that of the reference earmuff, this observation suggested that the phenomenon was not influenced by the assembly components of the earmuff. In this case, the IL dip was due to the resonance (pumping motion) of the ear cups, which is dependent on the design parameters and the leakages around the cushion pads. In another work, [PERSON] and [PERSON] [41] demonstrated that the presence of leakages could deteriorate the intended performance of an earmuff in the low-frequency range (\(<\)500 Hz). To improve the IL at this frequency range, a perfect seal could be a feasible solution. Unfortunately, it would be challenging to achieve zero leakage as the compliance of the cushion pad with the morphology of the temporal bone  
Figure 4: Insertion loss curves of the reference earmuff (dark grey), the Coir-PP earmuff (red), and the Coir/C-PP earmuff (blue) in pink 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 16 Hz.  
(around the pinna) may not be perfect. The pressure exerted by the headband could also be tuned to minimise leakages around the cushion pads of the earmuff. For example, [PERSON] et al. [42] showed that a higher headband force would provide a better sealing of the cushion pads, improving low-frequency performance of the earmuffs. However, the higher headband force would lead to a higher equivalent stiffness of the cushion pads, which could compromise comfort, discouraging the user from wearing the earmuff over a prolonged period of time as suggested by [PERSON] and [PERSON] [22].  
The second IL dip occurred at different frequency for the reference and the composite earmuffs. Similarity was only observed between the composite earmuffs. The IL dip occurred at around 1088 Hz and 736 Hz for the reference and the composite earmuffs, respectively. This observation could be attributed to the structural resonance of the ear cups. Elaborating further, the ear cups of the reference earmuff were made from ABS, which had a higher structural stiffness as opposed to the composite ear cups. Hence, a higher structural resonance would be expected for the ABS ear cups.  
Up to this point, the presented results suggested that the composite earmuffs would be ineffective for low-frequency noise reduction, which is the dominant source of noise exposed by the ground crew at an airport. However, it should be noted that the acoustical performance of the composite earmuffs were determined based on only the continuous noise environment (pink noise). In the study by [PERSON] [23], he showed that the acoustical performance of composite earmuffs may differ in transient noise environments. As such, the proposed composite earmuffs were further evaluated based on aircraft take-off exterior noise.