Effect of internal layer thickness on ultrasonic transmission through multilayer porous media

Normal-incidence ultrasonic transmission through a multilayer structure made of two fluid-saturated porous layers separated by a thin solid interlayer is studied theoretically and experimentally. The interlayer lies between the two saturated layers and its thickness is varied parametrically. Each porous layer is described either within the Biot single-porosity framework or within the Berryman-Wang double-porosity extension, and the multilayer boundary-value problem is solved using a global matrix method. Transmission coefficients are computed as a function of frequency for interlayer thicknesses between 0 and 7.5 mm. Increasing the interlayer shifts the frequencies of the transmission extrema and also modifies their amplitudes across the band. A predicted peak-frequency sensitivity S_f, defined as the frequency shift per millimeter of interlayer, reaches up to about 0.37 kHz/mm for the most responsive peaks. Measurements on water-saturated glass bead and Robu sintered borosilicate glass samples, with and without a 2.5 mm PVC interlayer, are compared with the model over the transducer passband. Both materials show agreement in the positions of the main maxima, the residual differences being in the depth of the spectral minima.