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

Abstract

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 δ(T)−2xN exp(−xNT)θ(T), where T=tc/a, the delta function is the initial specular echo, θ is a unit step function, and xN=ρa/ρEh is the dimensionless null frequency. The densities of the shell and water are ρE and ρ. Thus the mass-per-area ρEh 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.]