Experimental study of Lamb waves having a negative energy velocity in a thin plane plate immersed in liquid.

Abstract

Lamb waves having a negative energy velocity have been theoretically predicted; they are called negative waves for short. This word is used if the phase velocity is in the opposite direction to the energy velocity. When a plate is immersed in a liquid, this phenomenon still occurs. These waves become leaky as their energy is being reradiated into the liquid. Any negative wave has the same limit frequency as a Lamb wave, and the same label with a prime (′) sign (i.e., S2−S2′). Some common solids were studied: duralumin, stainless steel, brass, and glass. The negative waves were generated (as Lamb waves are) by a tone burst bulk wave and observed after propagation over the plate by translating one of the transducers in order to receive the negative waves. The existence of several negative waves was proved; especially the A3′ glass wave which can be observed quite easily. When the frequency × thickness product decreases, several properties were observed in accordance with the theory: (i) the absolute value of energy velocity increased and reached a maximum near the cutoff frequency, (ii) the attenuation increased. A time‐frequency study follows which enables the calculation of the energy velocity.