BACKSCATTERING OF CHIRPED BURSTS BY A THIN SPHERICAL-SHELL NEAR THE COINCIDENCE FREQUENCY

Previous ray calculations and observations [L. G. Zhang, N. H. Sun, and P. L. Marston, J. Acoust. Soc. Am. 91, 1862-1874 (1992); G. Kaduchak and P. L. Marston, J. Acoust. Soc. Am. 93, 224-230 (1993)] have shown that tone bursts with a carrier frequency ka close to the coincidence frequency of a thin spherical shell in water exhibit a backscattering enhancement. A distinct feature associated with the a0- guided wave was enhanced relative to the specular reflection by a factor close to 3.1. In the present research, a Fourier synthesis from the partial wave series is used to investigate how chirping the frequency of the incident burst influences the enhancement. An enhancement is still evident even when the incident burst is swept through the entire ka range of interest sufficiently rapidly for the specular and guided wave echoes to remain distinct. The enhancement is larger when ka is swept from high-to-low than from low-to-high. This is a consequence of a decrease in the group velocity with ka resulting in a localization of the echo for the high-to-low sweep. The enhancement can now exceed the aforementioned value of 3.1 and should increase the signal-to-noise ratio. Experimental records confirm the effect on a real shell.