## 1 Introduction
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; 20; 21; 22]. [PERSON] [23] 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; 8]. 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; 25]. 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; 29]. 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.