Prediction of Acoustical Performance with and Without Airflow Resistivity of Fibrous Natural Materials

Abstract

Traditional sound-absorbing materials for noise control are rockwool and glasswool which serve the purpose pretty well but are accompanied by some serious health hazards during their manufacturing or processing or application. Hence researchers and practitioners are coming up with new materials, particularly from natural sources, which apart from being cheap and readily available, have good noise absorption properties along with minimum effect on environment and health hazards. When sound is incident on these materials, some portion is reflected back (due to impedance mismatch), some portion is dissipated within the material (due to thermal and viscous loss in the pores) and the remaining is transmitted to the medium behind the material. Sound absorption is defined as dissipation plus transmission; hence, any parameter which increases any of these two effects will increase the sound absorption. Just like the synthetic sound-absorbing materials, these natural materials are also porous in nature which creates sufficient interest to explore them for potential acoustical applications. Several researchers have conducted acoustical investigations on materials which are used in some other form such as bamboo, jute, kenaf, hemp, ramie, sisal, coconut coir, tea leaf fibre, betel nut fibre and also on some waste materials like recycled rubber, cigarette butt, bagasse, oil palm empty fruit bunch fibre. In this chapter, a mathematical model has been developed to predict the normal sound absorption coefficient of jute and waste cotton. It can also be predicted by estimating the airflow resistivity of a material and feeding that into established models but the amount of error encountered in this approach is often large. So, with an objective to minimise the error, characteristics impedance and complex wave number of the materials have been experimentally estimated. With the minimum sum of squares of error curve fitting approach, a MATLAB code has been developed to obtain the modified coefficients in Delany–Bazley (D–B) model. These equations have been used to predict the sound absorption coefficients of Jute fibres and waste cotton, which is compared with the actual ones obtained by conducting experiments on impedance tube. The modified model was found to be of the greater degree of accuracy than the original D–B model. Further, the airflow resistivity term was eliminated in the Matlab code and the model was used to predict the normal sound absorption coefficient at different frequencies. The results of this model were compared with the result of the original D–B model and experimentally estimated normal absorption. It was also found to be closer to the actual behaviour than the one predicted by D–B model.