Acoustic wave transmission through sub-wavelength slits or holes, then on layers of periodically spaced cylinders and double fishnet structures, known as acoustic metamaterials has been studied in many works in relation with physical phenomena such as extraordinary resonant transmission, sound shielding etc. We present for the first time the study of analogous phenomena for Lamb waves propagating in a thin plate, in which symmetric plate mode S0 is considered. Rectangular holes are chosen due to the fact that more geometrical parameters can be varied comparatively to circular holes. In the aforementioned works the propagation medium is in general made of a fluid while the case of a solid host has been less considered. We have investigated rectangular air holes in a silicon plate and optimized the geometrical parameters in order to get a low frequency broad band attenuation. We show that, when a second row of slits is added, the choice of the distance between both rows allows for the realization of a broadband attenuation in the transmission up to 99%. With additional rows of slits we find the properties of a phononic crystal constituted by a periodic array of holes in a plate. These investigations should have implications for sound isolation and sensing applications. The work is supported by the Agence Nationale de la Recherche and Direction Générale de l'Armement under the project Metactif, grant ANR-11-ASTR-015.
