The intent of this passive noise control project is to develop a method for predicting sound absorption characteristics of heterogeneous fibrous materials such as needle punched nonwoven substrates. The research will be useful in producing more efficient sound absorbent materials and in designing materials for low frequency sound absorption. This is a three phase project comprising three objectives: 1) the development of an empirical model for predicting the effects of needle punched nonwoven substrate characteristics on sound absorption; 2) the theoretical determination of the mechanisms involved in the absorption of sound by heterogeneous substrates such as needle punched nonwoven fabrics; 3) the determination of the substrate design criteria necessary for low frequency absorption. The model development task is accomplished by systematic measurements of the sound absorption coefficients for a large number of needle punched fabrics. The model is an empirical relationship obtained by fitting a parametric expression to the measured sound absorption coefficient data using stepwise multiple regression analysis. The second objective is achieved through a three stage process comprising development of structural model of the needle punched nonwoven fabric, development of a dissipation model including a pore size distribution function, viscosity correction factor and flow resistance function for needle punched fabrics, and development of an acoustical model for needle punched fabrics. The third project objective is met by varying the model parameters to predict the maximum sound absorption at low frequencies.
