Measure of the thickness of a cylindrical shell with a focused beam

The acoustic scattering from a cylindrical or spherical object immersed in water and insonified with plane wave has been theoretically and experimentally studied. The backscattering spectra show minima which are related to the circumferential waves circumnavigating around the object. These waves are identical to the Lamb waves propagating in a plate. Two types of resonances are observed on ultrasonic spectra: the resonances related to the thickness and the resonances related to the stationary state that establishes around the cylindrical shell from the circumferential waves. To eliminate the second type of resonances a focused beam are used to excite only the thickness resonances. The cylindrical shell thickness is calculated from these resonance frequencies experimentally measured