Study of the resonances of periodic plane media immersed in water: theory and experiment.

The paper deals with the study of the resonances of 1D periodic media composed of N elementary cells formed with two perfectly bonded layers which exhibit a high acoustic impedance contrast. In the case of a periodic bilayer structure constituted of a fluid layer and an elastic plate, it was shown in previous theoretical works that additional modes appear compared to those of a single plate. These are called structure modes. At low frequency, the so-called vertical modes are found. Approximate expressions of their cut-off frequencies are given and their numerical values match with the exact ones. At high frequency, the Lamb type modes are degenerated and modes in the fluid layers are also observed. Preliminary experimental results have already proved the existence of such phenomena for one and two periods. In our work, an experimental validation has been performed in the case of N periods made with a glass isotropic elastic plate and a water fluid layer, where the number N ranges from two to five. A good agreement is shown compared to theoretical works.