Sound absorption properties of structures developed using waste materials

Abstract

This study investigates the sound absorption coefficients (SAC) of structures developed using coconut shell powder, charcoal powder, and hair. Large amounts of coconut shells and human hair are readily available in Sri Lanka. The study used those to build sound absorption structures (SAS) using natural liquid rubber as a binder. Sample sheets were prepared by varying the volume ratio of the binder and fillers. Samples were tested for their SAC by using the impedance tube for different frequencies within the range of 1000-3500 Hz. Finally, the samples were further improved by changing physical properties such as lowering density, increasing thickness, and increasing porosity with surface roughness and combining layers. A coconut shell powder sample with a 50% volume ratio shows the SAC in the range of 0.80-0.83 for 2500 Hz and 0.85-0.93 for 3000 Hz. Improved charcoal samples, with 1.5 cm in thickness and a volume ratio of 66%, exhibited sound absorption over 60% beyond 2000 Hz, which indicates efficient absorption of incident sound energy without reflections or transmissions. A Hair sample also showed some magnificent SAC of 0.93 at 2500 Hz. A 2 cm thick layering type sample, which combines hair sample layers and charcoal powder sample layers, also shows a SAC of 0.80–0.80 for higher frequencies more fabulous than 3000 Hz. Acoustic materials in the market (thickness > 2 cm) have SAC of 0.20-0.99 for 125-4000 Hz. So, these versatile, cost-effective, eco-friendly SAS are suitable for architectural acoustics, studios, theatres, and the automotive industry due to flexibility, easy preparation, and thin profile.