### ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

## 4pPAb4. Impulse response of thin shells: Source development, analysis of
the bipolar specular contribution, and computations showing the effect of water
on the inside of the shell.

**Gregory Kaduchak
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Philip L. Marston
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*Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814
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The response of shells to a delta function pressure impulse is well suited
to interpretation with ray methods. We examine the theory for PVDF sheet
sources [C. S. Kwiatkowski, G. Kaduchak, and P. L. Marston, J. Acoust. Soc. Am.
94, 1831 (A) (1993)] which facilitated wideband impulse scattering measurements
in modest-sized water tanks. Response features for spherical and nonspherical
shells will be discussed. An approximation is developed for the bipolar
specular feature which for an empty sphere of radius a becomes
(delta)(T)-2x[sub N] exp(-x[sub N]T)(theta)(T), where T=tc/a, the delta
function is the initial specular echo, (theta) is a unit step function, and
x[sub N]=(rho)a/(rho)[sub E]h is the dimensionless null frequency. The
densities of the shell and water are (rho)[sub E] and (rho). Thus the
mass-per-area (rho)[sub E]h for the shell of thickness h affects the magnitude
and decay time of the negative feature. Observations for an empty spherical
shell and an end cone piece of an MIT/NRL model shell will be examined.
Computations show that both the bipolar feature and the coincidence frequency
wave packet for spherical shells are not quenched when water is inside the
shell. [Work supported by ONR.]