Investigation of Sound Absorption Properties of Hybrid Multilayer Density Gradient Coir Structure

In this work, hybrid structures from coir nonwoven matts and snippets (finely shredded denim fabric) were developed for determining the sound absorption property. Multilayer density gradient structures were first developed by layering compressed needle punched coir nonwoven matts of increasing densities. The sound absorption behaviour of showed good absorption in the mid and high frequency range because of the increase in density through the thickness that led to better frictional loss of the sound wave. Noise Reduction Coefficient (NRC) of 0.3 was achieved from a gradient density structure comprising of coir fibres alone. Hybrid structure was developed by layering the density gradient coir structure with a sound facing layer (made up of 1mm snippets sprayed on the surface of coir sheet). Addition of snippets formed small sized pores on the surface of the coir sheet thus increasing the surface area. The thin panel like structure of the sound facing layer absorbed low frequency showing characteristic peaks but showed decreased absorption in high frequency. Snippets improved vibrational loss and increased low frequency sound absorption achieving NRC of 0.5.