Transduction and audible displays for broad band sonar systems

This thesis describes an investigation into the feasibility of using a purely audible display in conjunction with a wide bandwidth frequency modulated sonar for the detection and tracking of underwater targets. Special techniques are devised for the construction of transducer elements and arrays to operate over an extremely wide bandwidth and to allow the generation of an audible display which is self-sufficient and effective. Certain psycho-acoustic aspects of the display are investigated with particular reference to the detection and tracking of large distributed targets, such as shoals of fish. It is shown that a wide arc of illumination in the horizontal plane may be used while an accurate tracking capability is retained. This permits the use of a fixed transducer array, thus eliminating an expensive scanning mechanism which is common in more conventional forward-looking sonars for fish detection. The results of sea trials with a prototype version of the system are described. The sea trials have shown that, although arrays of low directivity are used, a useful operational range may be achieved with the system. A large number of fish shoals were detected and successfully tracked during the trials