Mode-Coupling in Solid/Liquid/Solid Trilayers

This paper deals with elastic wave dispersion in trilayers composed of two solid plates (glass or aluminum) separated by a thin water layer. Two symmetrical (identical top and bottom layers) and two asymmetrical trilayers are investigated. It is shown that the dispersion curves of a trilayer can be analyzed in terms of the dispersion in the decoupled constituent layers, provided that the appropriate boundary conditions are used and that mode coupling between the modes of the decoupled layers is taken into account. The experimental data obtained by means of a double transmission immersion technique confirm this analysis. The main conclusions reached are (i) that the normal propagation modes of the decoupled constituents determine the asymptotic behavior of the trilayer modes; and (ii) that successive mode coupling is responsible for the course of the trilayer modes. Further, numerical and experimental evidence is found that doublets are formed around the modes of the decoupled solid plates in symmetrical but not in asymmetrical trilayers. It will be shown that mode coupling between two identical modes of the top and bottom layer is responsible for these doublets. Plots of the modulus and the phase of the reflection coefficient of the trilayers, as a function of frequency, further elucidate the analysis.