Acoustic Target Strengths of Some Penaeid Prawns

Abstract

Equipment has been developed to facilitate measurement of the acoustic target strengths of single penaeid prawns arranged at various orientations to the insonifying beam direction.

The dorsal plane target strengths of several species of penaeid prawns in the length range 16±2 cm and for frequencies in the range 50-1200 kHz are described by:

T_s=-44.4-2.52 × 10^-3f

for the maximum target strength values observed, and

T_s = -51.2-1.68 × 10^-3f

for values of target strength averaged over the dorsal plane. T_s values are in dB ref. 1 m and f, the insonifying frequency, is in kHz.

Target strength values averaged over the central ±40º of the dorsal plane were essentially constant with frequency, at a value of approximately - 47 dB ref. 1 m.

For the target lengths and sound frequencies used the amplitude of the backscattered pressure signal varies rapidly with changes in target orientation and an ensemble of such amplitudes, measured at 200 kHz in the dorsal plane, is shown to be distributed approximately according to the Rayleigh distribution function.