Vertical mode for an immersed structure composed of two elastic layers coupled by a thin water film

We aim at evaluating the thickness of a thin water film, coupling two elastic layers. By a plane acoustic wave approach, we have shown the existence of a particular eigen-mode, called the vertical mode (V-mode), which strongly depends on the fluid layer. The water film thickness is expressed as a function of the V-mode cut-off frequency. The value of this particular frequency is experimentally obtained by a retrodiffusion and resonance spectra analysis. However, in practice, the acoustic waves propagate as a bounded beam, and it is necessary to show that the measurement accuracy remains compatible with a plane wave approach