DETERMINATION OF THE VISCOUS CHARACTERISTIC LENGTH IN AIR-FILLED POROUS MATERIALS BY ULTRASONIC-ATTENUATION MEASUREMENTS

The concept of viscous characteristic length is used to describe the a coustical behavior of fluid-saturated porous media in the high-frequen cy regime. A method to determine this parameter consists of measuring the wave attenuation in the high-frequency limit. This method has alre ady been used for porous materials saturated by superfluid He-2. It is tested in the case of air-filled absorbent materials in a frequency r ange of [50-600 kHz]. The thermal characteristic length is assumed to be known or measured independently. Two examples are presented. In the first one the method is usable and the viscous characteristic length Lambda is deduced from the high-frequency behavior of the attenuation per cycle. In the second example, an additional attenuation occurs at high frequencies and only an estimate of Lambda can be given. Neverthe less, the estimation appears to be rather accurate. The values obtaine d by this method are compared to those determined by a nonlinear fit o f the dispersion curves. (C) 1996 Acoustical Society of America.