Prediction of the acoustic field due to a point source over a porous layer (L).

Comparisons are performed between the results of a simple classic formulation for the acoustic field created by a point source above a porous layer and an exact description. A semi-infinite layer and layers of finite thickness are considered. It is shown that for layers of finite thickness and small flow resistivity the use of the classic formulation can lead to large errors close to grazing incidence.

Acoustical measurement of the shear modulus for thin porous layers.

Simulations performed with the Biot theory show that for thin porous layers, a shear mode of the structure can be induced by a point-source in air located close to the layer. The simulations show that this mode is present around frequencies where the quarter wavelength of the shear Biot wave is equal to the thickness of the samples and show that it can be acoustically detected from the fast variations with frequency of the location of a pole of the reflection coefficient close to grazing incidence. The mode has been detected with this method for two reticulated plastic foams.