Analysis of effect of thickness and surface treatment on sound transmission loss characteristics of natural fibres

Noise pollution, driven by rapid urbanisation and city expansion, has created a growing demand for innovative and bio-degradable sound absorption materials. Traditional solutions such as synthetic acoustic foams are widely used due to their efficiency and low cost but raise environmental concerns because of their non-biodegradable nature. This study explored the use of natural fibres—coir and sponge gourd—as environmentally friendly alternatives for sound insulation. The research focused on the effect of fibre thickness and surface treatment (using sodium hydroxide (NaOH)) on their acoustic performance. The Fourier Transform Infrared Spectroscopy (FTIR) technique was used to understand the effects of the treatment on the functional groups of the fibre. The surface modification of the fibre surface was studied using an optical microscope, Brumauer–Emmet–Teller (BET) analysis and by analysis of Scanning Electron Microscope (SEM) images. An impedance tube setup was used to measure the sound transmission loss for both the untreated and treated fibres. The results showed that increasing the thickness of both coir and sponge gourd fibres improves transmission loss at lower frequencies but reduces effectiveness at higher frequencies. Surface treatment had a material-dependent effect: sponge gourd fibres showed improved transmission loss due to increased surface roughness and airflow resistivity, whereas coir fibres exhibited a decline in performance after treatment. These findings contribute to a better understanding of how natural materials can be optimised for acoustic applications through structural modifications.