A preliminary study of effect of air gap on sound absorption of natural coir fiber

Abstract

Porous fibers are widely used in noise control engineering to absorb sound energy. Natural fibers such as coir fibers have high potential to be used as an acoustic material. Utilization of natural fibers can be a great economic advantage since they are easily obtained as an agricultural waste. It was found that coir fiber has a favourable absorption at high frequency. Increasing the sound absorption at low frequency can be a great contribution to noise control engineering. Air gap or perforated facing is usually used to enhance the absorption at low frequency. This paper investigates the effect of air gap on noise absorption of coir at low frequency range. A panel with 20 mm air gap at three different thicknesses of 20 mm, 35 mm and 50 mm are prepared for noise absorption test using impedance tube. Absorption coefficient of the porous material is initially calculated using Delany-Bazley equation and then verified by the experimental measurements. Experiments are conducted in impedance tube for the three samples within the frequency range up to 5000 Hz. Experimental results shows that the maximum absorption coefficient for three thickness layers without air gap varies from 0.6 to 0.95 within frequency range of 1100 Hz to 5000 Hz. Addition of air gap moves the absorption towards low frequency range around 1000 Hz to 1500 Hz with almost same absorption coefficient for that three thickness layers. These results indicate that sound absorption at lower frequencies can be enhanced with air gap or perforated facing. However, resonances of the material can not be predicted using this model. Results also indicate that Delany-Bazley equations can justify the overall trend of absorption coefficient of coir fiber perfectly as demonstrated in this study.