Determination of the acoustic back-scattering patterns and cross sections of fish

Using a scale-model technique, the back-scattering polar diagrams of small fish in water were determined at 1.48 Mc/s, 625 kc/s and 360 kc/s over a wide range of fish length. As the length increased, the acoustic back-scattering cross sections in the reference directions undulated in a regular fashion, indicating interference between the signals from various parts of the fish. From the results, using suitable scaling factors, the acoustic cross sections of an isolated fish can be calculated over wide ranges of size and frequency. The ratios of the signals in different directions and of side-lobe amplitudes are also determined for fish of various lengths. The acoustic cross sections of a few simple bodies are amenable to calculation and, on comparison with the observed experimental trends, the part of the fish giving the largest signal at a given fish length can be inferred, as well as the effect of variation of frequency. The latter is checked by comparing the results at 625 kc/s with those at 1.48 Mc/s. The results are correlated with those of other workers who used large fish. Prior observations (Cushing and Richardson 1956) of the ratios of signals received from a fish at different frequencies can now be explained.