Inverse Acoustic Characterization of Rigid Frame Porous Materials from Impedance Tube Measurements

We will present a method for the inverse characterization of rigid frame porous materials using audible frequency acoustic measurements in a 3cm diameter impedance tube. We recover the six acoustical parameters of the Johnson-Lafarge model, namely porosity, tortuosity, viscous and thermal characteristic lengths and flow and thermal resistivities. The proposed method is based on a minimization process, where the quantities of interest are found as the minimizing values for the difference between measured and modeled density and compressibility. A scattering matrix formulation is used to obtain the reflection and transmission coefficients R and T, found as the elements of the scattering matrix, namely S11 = R and S12 = Texp(ikL), and to obtain the effective density and compressibility in the range of 200 - 6500 Hz. Five different porous materials with flow resistivity ranging from 2000 to 60000 Ns/m4 are tested and the results of the inversion process are compared to direct measurements of the acoustical quantities as well as to an already established recovery method developed by Olny and Panneton. The results are also validated against measurements with a rigid backing. It is found that the proposed method can be used to recover the material parameters quickly and reliably.