Use of a nearfield array for acoustic backscattering cross‐section measurements

Abstract

It is theoretically demonstrated that a large circular piston in an infinite rigid baffle operating at high frequency can be used as a nearfield array (NFA) to measure farfield backscattering (sonar) cross sections. Pulsed experiments are described which use the same piston both to insonify the object and receive the scattered wave. In transmitting, the piston produces a collimated plane wave within its Rayleigh zone. When the scatterer is placed within this zone, the scattered wave is received by the piston and integrated to reproduce a voltage response proportional to the scatterer's farfield amplitude function (NFA reciprocity principle). Measurements have been made on spheres and cylinders as well as model ships, insonified by a 4.75‐in. circular beam at a frequency of 15 MHz. The sonar cross sections (target strengths) obtained are in good agreement with those theoretically predicted by finite beam insonification scattering theory. Comparison of scaled model ship measurements with full‐scale target strength values shows reasonable agreement as well. [Work supported by NAVSEA‐034.]