ASA 126th Meeting Denver 1993 October 4-8

3pPA3. Time-frequency spectrograms of impulse scattering by shells: Quantitative comparisons with ray theory for Lamb wave contributions.

D. H. Hughes

815 Main St., Boonville, MO 65233

P. L. Marston

Washington State Univ., Pullman, WA 99164-2814

The Wigner distribution function (WDF) and quantities derived from it were used to investigate high-frequency scattering processes for a spherical shell in water [D. H. Hughes, Ph.D. thesis, Wash. State Univ. (1992)]. The investigation includes: (i) display of Gaussian smoothed WDF for ka as large as 500 with smoothing functions of different time and ka resolution; (ii) measurements of the leaky Lamb-wave decay rates from the smoothed WDF for different ka regions showing good agreement with ray theory based on complex partial-wave index poles; and (iii) evaluation of a derived quantity, the local temporal variance of the WDF as a function of frequency, showing how the temporal spread changes near a resonance [D. H. Hughes and P. L. Marston, J. Acoust. Soc. Am. 94, 499--505 (1993)]. The temporal variance at resonance was in general agreement with the ray theory predictions for the dominant scattering contribution. Structure in the smoothed WDF was also investigated for the prompt backward wave contribution that causes a prominent scattering enhancement at high frequencies. [Work supported by ONR.]