A method of calibrating a harbor for focusing low frequency sound

The objective of this work was to explore alternative methods to time-delay beamforming for focusing a sound field in a shallow water harbor environment. The approach is based on optimizing the phase of individual sources in an array to provide maximum sound pressure levels at a particular location in the harbor. The optimization scheme requires a priori interrogation of the harbor with a number of hydrophones in the area of interest. However, it is designed to be insensitive to vertical or horizontal boundaries and other time-independent obstructions that can be found in typical working harbors. Numerical simulations for classic array theory and the new Optimal Phase Search (OPS) method are compared and experimental data for both methods is presented. The field work investigated the extent to which low-frequency acoustic energy could be focused in very shallow water and was carried out in a semi-protected harbor in June 2008. Sound pressure levels generated with the new optimal phasing scheme were comparable with those based on classic array theory for the experiments, but the OPS method gave an increase in the SPL relative to the array method off-axis in the numerical simulations.