Characteristic dimensions and prediction at high frequencies of the surface impedance of porous layers

The surface acoustic impedance of a glass wool and a reticulated foam is measured in a free field up to 20 000 Hz. The characteristic dimensions Λ and Λ′ can be calculated for the glass wool, and the surface impedance can be predicted with no adjustable parameters. The motion of the frame is not taken into account. The agreement between measurement and prediction is good. For the foam, the characteristic dimensions cannot be calculated, because the geometry of the frame is not simple. A correct choice of Λ and Λ′ allows a precise prediction of the surface impedance for a large range of frequencies and different thicknesses.