## 1 Introduction  
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; 2; 3; 4; 5; 6; 7; 8]. 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; 11]. 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, [PERSON] 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 [PERSON] et al. [13], which instead focused on the Americans. [PERSON] [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 [PERSON] 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, [PERSON] 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.