Measuring the directivity index of underwater sound projectors

The directivity index Diof an underwater sound source usually is approximated because of the difficulty involved in performing the required power measurements and integrating over all angles. A simulation study conducted at the Underwater Sound Reference Division resulted in a model of a Dimeasuring system that minimizes the complexity of measuring the power profile about a sphere with accuracy to within ±0.1 dB. A system was assembled in accordance with the model and experimental measurements have been made in a shallow lake. The instrumentation and measurement techniques are presented in this paper. A complete three-dimensional power profile can be obtained in approximately 10 min running time with conventional hardware. Measured values of Diare in close agreement with theoretical calculations based on a perfect piston raditors, except for patterns with significant sidelobes.