Ultrasound reflection (R) and transmission (T) from an immersed porous plate of QF-20 (a registered trademark of Filtros, Ferro Corporation) are investigated. Assuming open pores boundary conditions for the theoretical computations, values of the physical constants given by Johnson et al. [D. L. Johnson, D. L. Hemmick, and H. Kojima J. Appl. Phys. 76(1), 115-125 (1994)] are used at first. Comparisons of R and T with experimental results show a discrepancy. It is then assumed that visco-elastic losses exist inside the solid part. When a small imaginary constant part is considered for the bulk moduli, a better fit is obtained between theoretical and experimental values of /R/ and /T/. However, the numerous and very close peaks prevent easy measures of the resonance amplitudes and widths. The transition terms built up from the linear combinations R-T and R+T allow these peaks to be separated. Comparisons between theory and experiments are made. This work validates, in the frequency range where the agreement is good, a method for the location of the symmetrical and antisymmetrical modes of the plate. A discrimination is also possible between modes resulting from the fluid phase motions and from the solid phase motions.

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This paper is devoted to the study of water-saturated porous plate/water layered structures by means of the transition terms defined from the reflection and transmission coefficients. Transition terms are obtained from the eigenvalues of the scattering matrix of the water-immersed structure and exhibit the symmetric or antisymmetric resonances of the structures. The N porous plates associated in our structures obey Biot's theory. The reflection and transmission coefficients of a unique water-saturated plate being calculated, an induction on N process allows to find the reflection and transmission coefficients of a given N plate/water-layer structure. The plates used in the experiments at normal incidence are 5mm thick. The reflection and transmission coefficients of sets of 1, 2, 3, and 4 water immersed plates, separated from each other by a 1cm water gap, are measured. There are good agreements between the calculated and experimental transition terms. Which obey the Breit-Wigner resonant form which characteristics can be obtained.