Search

A matrix representation of the acoustical properties of porous layers at oblique incidence is used to predict the surface impedance of layered porous materials with impervious screens. The screens can be located inside the material or at the surface. Comparisons between prediction and measurement achieved with a new technique are given.

Biot has shown that two compressional waves can propagate in a porous material with an elastic frame. For a material with a heavy frame and a medium flow resistivity, one of these waves propagates mainly in air, and the other is created by frame oscillations. The amplitude of this wave, which is generally negligible, can be important at the λ/4 resonance of the frame. A fast variation of the surface impedance appears at this resonance. This effect, which is predictible with the Biot theory, has been observed on a glass wool of high stiffness.