Hydroacoustic Impulse Generator

Abstract

A repeatable generator of unidirectional strong underwater acoustic transients is described. The method of transient generation involves a three-step process in which (1) sea-pressure energy is converted to kinetic energy of fluid adjacent to a moving (sea pressure driven) piston; (2) the kinetic energy is converted to an acoustic shock upon near-instantaneous impact of the piston upon an anvil; and (3) the acoustic shock propagates backwards along the piston axis with a pressure-time history and directional pattern that is dependent upon the geometry of the piston and its adjacent baffle. The results of measurements on a small-model impulse source are described and are compared with theoretical predictions. This model source embodies a 3-in.-diam piston and produces peak acoustic pressures on axis of magnitude greater than 130 dB re 1 μbar at 1 yd at depths of the order of 14 ft. It is shown how the shape of the piston contour can affect the shape of the pressure-time history on axis, the shape of the transmitted spectrum, and directivity of the peak radiated signal. Theoretical scaling predictions suggest that it should be possible to produce peak acoustic pressures of the order of 170 dB re 1 μbar at 1 yd with an energy spectrum centered in the low kilohertz range. [Work supported in part by the U. S. Office of Naval Research.]