OBSERVATION OF THE MIDFREQUENCY ENHANCEMENT OF TONE BURSTS BACKSCATTERED BY A THIN SPHERICAL-SHELL IN WATER NEAR THE COINCIDENCE FREQUENCY

A prominent feature predicted for the backscattering of tone bursts by thin spherical shells is an enhancement of a guided wave contribution in the midfrequency range [ L. G. Zhang et al., J. Acoust. Soc. Am. 91, 1862-1874 (1992) ]. This feature may be useful for certain inverse problems and is associated with a strongly coupled slightly subsonic wave denoted by some authors as the a0- wave. The present research gives a comparison between a my approximation and experiments in which tone bursts having carrier frequencies in the range 35 < ka < 70 were incident on an empty stainless steel shell in water of radius a. The sphere's radius to thickness ratio is a/h = 43.8. Time records of echoes provided a means for measuring the guided wave contribution distinct from that of the specular reflection. As predicted, the guided wave echo can be over three times the amplitude of the specular echo and the ka dependence generally follows the predicted hump. Echoes manifest structures similar to those calculated by a time domain Fourier synthesis from the partial wave series.