Free‐field acoustic calibration of long underwater acoustic arrays in a closed tube

Abstract

A technique is described for measuring the complex free‐field sensitivity and directivity patterns of long underwater acoustic arrays in a closed tube. The tube contains sound projectors and monitoring hydrophones located along its length. Peforming these free‐field calibrations normally requires a large body of water to insure that the reflections from the sides, surface, and bottom have a negligible effect on the measurement. The projectors are driven to produce an acoustic pressure in the tube that is equivalent to a free‐field plane acoustic wave propagating from any angle of incidence. The required projector drives, both amplitude and phase, are determined by measuring the transfer function for every projector and every hydrophone location and inverting the resulting transfer matrix. A prototype 10‐m‐long system was developed. The system operates over the frequency range from 100–2000 Hz and is capable of simulating depths to 600 m. Directivity patterns, obtained using the prototype, for a test line array 5 m long were in good agreement with theoretically predicted patterns. [Work supported by ONT.]