Backscattering of a short ultrasonic pulse from thin cylindrical shells

We study the backscattered signal from thin cylindrical shells irradiated by short ultrasonic pulses. Various materials (duraluminum, stainless steel, copper) and various thicknesses of the shells are considered. The shells are filled with air and immersed in water except in one experiment where the surrounding medium is ethanol. From the experimental results, we deduce the variations of the group velocity of the waves circumnavigating the shell. Two such waves are observed experimentally and their dispersion curves are compared with those of the waves propagating in plates immersed in the same fluid.