A Review of the Factors that Influence Sound Absorption and the Available Empirical Models for Fibrous Materials

There is a drastic need in industries, such as the automotive and construction industry, for easy-to-use empirical models that can effectively and efficiently predict the sound absorption coefficient of fibrous absorbers. Hence, this paper presents a review of the factors that influence sound absorption, such as bulk density, composite thickness, porosity, fibre diameter, airflow resistivity, tortuosity and surface impedance. Also, the available empirical models for the prediction of the sound absorptions coefficient of porous fibrous materials and their working range are presented. These models have been tested using experimental data, based on natural fibres such as coir, kenaf, hemp and sheep's wool, in order to test their accuracy. An analysis of the data is presented. It is seen that the present empirical models available are not adequate for the prediction of the sound absorption coefficient for the natural fibres presented, and hence, new models that can accurately predict the sound absorption coefficient need to be developed. The current models available are also seen to be extremely sensitive to composite thickness and bulk density changes. However, they are able to give the designer a good starting point. Therefore, this review is intended to be a comprehensive and valuable reference source for designers of sound absorbing materials in order to save time and costs during the design phase. Therefore, this contribution will enable the designers to make quick informed decisions on the type of absorber to use for a particular application as well as allow for quick selection of an appropriate empirical model for accurate prediction of the sound absorption coefficient of the selected fibrous porous material.