Acoustic Absorption Characterization and Prediction of Natural Coir Fibers

The remarkable properties of natural lignocellulosic fibers such as biodegradability, light weight, low density, low cost and non-toxicity as well as being an alternative to sound absorbers made of synthetic fibers have attracted many researchers in the field of acoustics. The purpose of the present study was to compare the estimated sound absorption coefficient of composite samples made of natural coconut fibers by using empirical models and comparing them with the results of experimental data. The normal sound absorption coefficients of the samples were measured with an impedance tube. The samples were fabricated in three different thicknesses (25, 35 and 45 mm) with air gaps behind them and had a constant density of 200 kg/m(3). Next, calculations were made to estimate the absorption coefficients of the samples by coding in MATLAB and using the differential equation algorithm along with Delany-Bazley, Miki and Johnson-Champoux-Allard models. Based on the results, the sound absorption coefficients of the samples increased significantly with increasing frequency. Additionally, increasing the thickness of materials at constant densities increases the absorption of sound, especially at lower frequencies (<1000 Hz). Comparison of the experimental data and estimations of the models showed that by increasing thickness, the predicted acoustic absorption coefficients for the samples become closer to the data from the experimental tests. At frequencies <1000 Hz, increasing the air gap at the back of the sample to 3 cm would elevate the values of sound absorption coefficient. The samples made of coir fibers would effectively dissipate the energy of sound waves. It is noted that increasing the absorption of the sound in such materials is related to the longer depreciation process of thermal and viscous transfer between the air and the absorbing materials in the composite.