ABSTRACT
In order to predict the acoustical behavior of noise protecting devices including porous materials, it is necessary to determine the intrinsic acoustical parameters of these heterogeneous media. If accurate measurements of static permeability and porosity can be done using non-acoustical techniques, measuring tortuosity and, viscous and thermal characteristic lengths, and “thermal permeability”, remains difficult for a wide range of porous materials. We present here a new inverse method, based on the prior knowledge of static permeability (or resistivity) and porosity, and the acoustical measurements of the equivalent dynamic permeability (or density) and bulk modulus of the material, using a standing waves tube. Assuming a motionless skeleton for the porous material, the determination of these two complex valued functions is sufficient to characterize the wave propagation in the fluid network. Moreover, visco-inertial and thermal effects are perfectly separated. Then, using the semi-phenomenological models, introduced by Johnson et al, and Lafarge-Allard-Champoux, for respectively the dynamic density and compressibility, we propose analytical solutions for calculating the unknown parameters over the frequency range of measurement. It is then possible to check that these parameters do not depend on frequency. Experimental results obtained for very different materials are presented, and reveal the efficiency and simplicity of the method.
