Leaky backward Lamb waves in various isotropic and anisotropic plate/liquid systems

The basic characteristics of leaky backward Lamb waves are investigated theoretically and numerically in various combinations of isotropic (aluminum, steel) and anisotropic (YX-NBBT95/5, YX-LiNbO
₃
) plates and surrounding liquids (water, petrol, liquid helium). The phase velocity and attenuation, the group and energy flux velocities, the energies and energy fluxes are among the studied characteristics. It is proved in general anisotropic piezoelectric case that (i) the total (integrated over the depth of the plate and in the liquid) time-averaged energy flux for the leaky backward Lamb waves is identically equal to zero, (ii) the total (integrated over the depth of the plate and in the liquid) time-averaged kinetic and potential energies are not equal to each other, (iii) the energy flux velocity is not equal to the quantity commonly referred to as the group velocity. It follows from our analysis that anisotropy of plates gives a new opportunity to reduce the losses caused by the leakage up to zero at the points of thickness resonances and the backward modes appearing, contrary to isotropic plates where such losses are unavoidable.