## 2 Materials and Methods
### Experimental Details  
The experiment was performed in a reverberation chamber of 226.9 m\({}^{3}\) with reference to ANSI S12.42 [33] and BS EN ISO 4869-3 [34]. A sound quality head and torso simulator (Type 4100, Bruel & Kjaer, Naerum, Denmark) and a data acquisition unit (Type 3663, Bruel & Kjaer, Naerum, Denmark) were placed on a table, which was positioned at the centre of the chamber. Pink noise (50-12,800 Hz) was generated and amplified by a noise generator (Type 1405, Bruel & Kjaer, Naerum, Denmark) and a signal amplifier (BAS002, Larson Davis, Depew, NY, USA), respectively. The signal was then transmitted by an omnidirectional loudspeaker (BAS001, [PERSON], Depew, NY, USA) situated at one corner of the chamber. An additional microphone was placed at 1 m away from each side of the simulator's moulded pinna. These additional sensors served to ensure consistency in the sound field with reference to the sound pressure level (SPL) recorded by the simulator without any earmuff (open ear). Figure 3a shows the experimental set-up in the reverberation chamber. It should be noted that the simulator was designed for evaluating sound quality in vehicle cabins and other optimisation studies [35]. As such, the results obtained from the simulator were only valid \begin{table}
\begin{tabular}{c c c} \hline
**Ear Cup** & **Mass (g)** & **Estimated Cost (5/kg) \({}^{*}\)** \\ \hline Reference & 39.0 & 5.00 \\ Coir Fibre-Reinforored Polypropylene & 27.6 & 3.00 \\ Coir/Carbon Fibre-Reinforored Polypropylene & 28.1 & 8.00 \\ \hline \end{tabular} \({}^{*}\) Note: Amount is in Singapore dollar.  
\end{table} 
Table 1: Mass and estimated cost of the respective ear cups. Figure 3: (**a**) Overview of the experimental set-up in the reverberation chamber with a reference earmuff worn on the simulator; and (**b**) closed-up view of the simulator with the composite earmuff worn based on the reference scales. \begin{table}
\begin{tabular}{c c c c} \hline
**Material** & **Type** & **Density (g/m\({}^{2}\))** & **Source** \\ \hline Coir Fibre Fabric & Non-Woven & 120 & Can Tho University, Can Tho, Vietnam \\ Carbon Fibre Fabric (282 3K) & Plain Woven & 197 & Hexcel, Stamford, CT, USA \\ PP Film (Cosmoplene Y101E) & Unmodified & 90 & The Polyolefin Company, Singapore \\ \hline \end{tabular}
\end{table} 
Table 2: Details of the materials used. as relative measurements for benchmarking of the composite earmuffs with the commercial earmuff. [PERSON] et al. [PERSON] et al. (2017) highlighted the same limitation in a recent study. Therefore, the presented results must not be misunderstood as a direct indication of real-ear attenuation values, which is one of the most accurate evaluation methods for HPDs [PERSON] et al. (2017). The subsequent stage of the study involved the consideration of aircraft take-off exterior noise to emulate the noise field exposed by the ground crew at the airport. It should be noted that the use of a reverberation chamber to emulate the aircraft take-off exterior noise environment is a limitation of the present study. An anechoic chamber would better emulate the scenario albeit such a facility was not available. The recorded soundtrack--downloaded from the Internet--was played via an audio system (ZS-RS70 BT, Sony, Minato, Tokyo, Japan). Due to technical limitations, the omnidirectional loudspeaker was substituted with a pair of self-powered loudspeakers (DXR15, Yamaha, Hamamatsu, Shizuoka, Japan), which were placed at different corners of the chamber. Comparing the simulator and a real human head, it may be understandable that leakage paths exist differently in both cases. In view of this concern, efforts were made to minimise possible leakage paths that may exist in the simulator and reduce experimental uncertainty. For instance, prior to each set of measurements, the simulator was inspected to ensure that the moulded pinnae remained well-fitted in the respective recesses of the simulator.