Determination of the acoustic scatter patterns and cross sections of fish models and ellipsoids

Abstract

At a frequency of 1.48 Mc/s, measurements are made of the acoustic back-scattering polar diagrams of models of a fish body and of ellipsoids, in various materials, some opaque and others translucent to sound. From these diagrams the signal level scattered from the fleshy body of a fish can be deduced as distinct from the previously measured signal from the complete fish, which included additional contributions from swim bladder and vertebral column. Analysis of the echo from a fish and calculations are thereby facilitated. Results for the models, the ellipsoids and fish are compared in terms of signal levels, reflectivities of the materials, shapes of the polar diagrams, numbers of lobes therein and their relative amplitudes. An ellipsoid is found to be a good approximation to the fish body in regard to acoustic properties if its main axes are identical with those of the fish (less fins and tail) and if it is made from a material having the same reflectivity as fish flesh. As many of the acoustic effects associated with an ellipsoid are amenable to calculation, this approximation is important. The signals scattered by real fish differ substantially (particularly in magnitude) from those scattered by the models, which do not contain a swim bladder, bones, etc.