Lamb waves in sandwich plates: group velocity not equal to velocity of energy transport

Since Biot’s first work on acoustic waves in non-dissipative solid media, it has been known that the group velocity of bulk acoustic waves coincides with the velocity of wave energy transport. The recent studies on these types of velocities for electromagnetic waves reveal that (i) these can differ, and (ii) the superluminal group velocities may exist. The current research demonstrates that in the case of Lamb waves propagating in sandwich clamped–clamped plates, the group velocity can exceed the largest longitudinal bulk wave velocity, and, moreover, the group velocity can be infinitely large, similarly to electromagnetic waves.