A wave model for rigid-frame porous materials using lumped parameter concepts

The work presented in this paper concerns the behaviour of porous media when exposed to a normal incidence sound field. A propagating wave model based on lumped parameter concepts of acoustic mass, stiffness and damping is used to investigate the absorption phenomena due to the wave propagation in the layer(s) and interference effects due to the wave reflection-transmission at the interfaces of the layer(s). Results from the theoretical model have been validated by measurements on samples of consolidated rubber granulate material. Two typical installations where a layer of porous material is placed next to a rigid wall, and where it is placed at a distance from a rigid wall are used as reference cases. The geometrical and physical properties of porous materials can be described by such parameters as the non-dimensional shape factor and the porosity. The propagating model introduced is used to investigate the effect of these two parameters on acoustic absorption and thus relate the physical properties to the acoustic behaviour.