No abstract

A new method for measuring impedance has been used for evaluating the normal surface impedance of three foams in free field. The results have been interpreted using the Biot theory, the two dilatational waves being taken into account. It has been pointed out from the theory that, for high flow resistance, the ratio of the acoustical velocities of the frame and the air is close to 1 at the surface of the foam. This ratio decreases with flow resistance but is never negligible for the studied foams. For foams with high flow resistance, the contributions of the two waves must be taken into account when calculating the impedance, and a description with only one wave would not be realistic. For foams with medium flow resistance, the one-wave approximation for calculating the surface impedance is a good approximation in the whole range of acoustical frequencies.

This paper provides a comprehensive review of the propagation of ultrasonic waves in anisotropic porous materials. The equivalent fluid model (or Allard-Johnson theory) which is relevant for air-saturated porous media is described. It takes into account viscous and thermal losses occurring during the movement of the fluid within the motionless solid frame. When the skeleton is moving as well, the coupled Biot theory should instead be used. This theory becomes intricate when anisotropy is considered due to a very large number of physical parameters to be determined. A strong formal correspondence between the anisotropic Biot wave and the thermal wave of dynamic thermoelasticity in non-porous media is outlined. Standard ultrasonic methods. generally used at low frequency (i.e. 20-500 kHz) are very effective in order to characterize anisotropy in porous media. Both reflection and transmission configurations have been used. Special attention has been devoted to the measurements of the anisotropic tortuosity, but also to the viscous and thermal characteristics lengths. Finally, some inverse problems related to these measurements are solved and others. which are still open, are presented. Q 1998 Elsevier Science B.V.

No abstract