Measurement and interpretation of the impulse response for backscattering by a thin spherical shell using a broad-bandwidth source that is nearly acoustically transparent

Abstract

A novel source was developed to produce a plane-wave unipolar pressure impulse with a wide range of frequency components. The source consisted of a PVDF sheet with water in contact with both sides. The PVDF was driven by a step voltage. This source is nearly acoustically transparent and was used for backscattering from an empty stainless-steel spherical shell. The shell was placed in the near field of the source where it experienced a plane-wave pressure impulse followed much later by edge contributions resulting from the finite source size. A hydrophone was placed in the far field of the scatterer on the opposite side of the source. Prominent features in the shell’s calculated impulse response are observed over a wide frequency interval. Time records reveal an approximately Gaussian wave packet from the excitation of the subsonic a0− wave associated with the backscattering enhancement near the coincidence frequency (≊309 kHz). Superposed on the same records are large contributions from the low-frequency excitation of the a0− wave and from the s0 wave. A bipolar feature of the initial response was found to be associated with the finite inertia of the shell and the null frequency. An approximate theory predicts that the associated relaxation time depends on the mass per area of the shell and the density and sound speed of the surrounding water. The shell used in the experiment has a thickness to radius ratio of 2.3% and the scattering phenomena of interest occur between 8 and 450 kHz corresponding to ka from approximately 1 to 70. The 10-kHz ringing of the target associated with the a0− wave is quite pronounced.