ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

1aPA7. Pressure impulse response measurements for elastic scatterers in water and regulation of the source spectrum.

Philip L. Marston

Scott F. Morse

Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814

A large PVDF sheet adjacent to the scatterer has been used to excite the plane-wave pressure impulse response of elastic shells over a wide spectral range [Kaduchak et al., to be published in J. Acoust. Soc. Am. (1995)]. Since the sheet is transparent to the scattered signal, the receiver hydrophone may be on the opposite side of the sheet. The understanding and control of the spectrum of the radiated pressure pulse is useful for facilitating the detection of specific target spectral features. An idealized circuit model is examined where the sheet is excited by a step voltage source having a real impedance given by the resistance R. Let C denote the sheet capacitance and let the step occur at t=0. The radiated pressure for a thin sheet becomes a step followed by an exponential tail: exp(-t/RC). The magnitude spectrum is proportional to [1+((omega)RC) [sup 2]][sup -1/2]. Increasing R reduces the high-frequency content which may be advantageous for certain measurements. Applications include the impulse excitation of low-frequency structural resonances of shells in water. Adjacent modes exhibit a prominent beating in the response. [Work supported by the Office of Naval Research.]