Ultrasonic Measurement of Tortuosity and Viscous Characteristic Length of Double-Layered Porous Absorbing Materials with rigid frames.
Résumé
We propose an indirect method for measuring simultaneously acoustic parameters describing the ultrasonic propagation in double-layered porous material. The porous media consist of two slabs of homogeneous isotropic porous materials with a rigid frame. Each porous slab is described by an equivalent fluid model, in which the acoustic wave propagates only in the fluid saturating the material. The inverse problem is solved numerically using experimental transmitted waves in time domain. The direct problem is solved in frequency domains. Four parameters are inverted : tortuosity and viscous characteristic lengths of the two layers. Tests are performed using industrial plastic foams. Inverted values of acoustic parameters are close to those measured by conventional methods. Experimental and numerical validation results of this method are presented, which show the advantage of using the transmission for measuring the characteristic lengths, unlike the reflection.