Underwater-nearfield acoustical holography using a robotic controlled scanner

Abstract

A computerized, experimental facility has been developed to implement nearfield acoustical holography in a large underwater tank. The facility contains a robotic controlled hydrophone arm which allows the precise positioning and motion of a hydrophone in the tank completely under computer control. Automated scanning is done close to,the surface of a radiating/scattering object and the resulting pressure data are stored digitally. The frequency range of operation is dc to above 30 kHz. Using the principles of nearfield acoustical holography [Williams and Maynard, Phys. Rev. Lett. 45, 554 (1980)] this data are forward and backprojected to study the radiated/scattered acoustic fields. Forward projection provides directivity patterns, and intensity and vector-velocity field mappings. Backward projection provides surface velocity and pressure (fluid loading)—reconstructing the field in the extreme nearfield. Experimental results are shown providing a complete description of the field radiated underwater from a point-driven rectangular plate. Shown are its surface velocity, intensity vectors in the extreme near field, fluid loading effects. radiation efficiency, and directivity patterns.