J. Stuart Bolton
1077 Ray W. Herrick Labs., School of Mech. Eng., Purdue Univ., West Lafayette, IN 47907-1077
In recent tests it was found that the measured surface normal impedance and transmission loss of limp, fibrous materials could be predicted more accurately when using an elastic porous material model rather than a rigid porous material model: i.e., the motion of the solid phase was found to be acoustically significant. However, it was noted that if the frame stiffness was set to too small a value in the elastic porous model, the boundary value problem that must be solved to yield the surface impedance or transmission coefficient becomes singular. Thus it was of interest to develop a porous material model in which it is assumed from the beginning that the solid phase may move, but that it has no bulk stiffness. Under these circumstances, the Biot model for elastic porous materials may be simplified to yield a single second-order wave equation governing the propagation of a single wave type in the limp porous material. The solutions to that wave equation in combination with appropriate boundary conditions may then be used to predict either the sound absorption by or sound transmission through layers of limp fibrous material without concerns regarding the numerical stability of the predictions.