Nearfield acoustical holography using an underwater, automated scanner

A computer‐controlled, three‐axis Cartesian scanning facility has been constructed in a large water tank to provide accurate preprogrammed contouring with hydrophone probes. As the scanner moves on the preprogrammed contours, usually located very close to a radiating object, the pressure field is sampled at discrete points until a two‐dimensional (2D) pressure map is obtained. This pressure map is essentially a hologram containing amplitude and phase information which can be processed with a computer using a technique called nearfield acoustical holography (NAH). This processing provides the pressure, vector velocity, and vector intensity anywhere in the space from the surface of the source to the farfield. Backward projection (reconstructing the field at the source surface) provides surface velocity and pressure (fluid loading) on the actual source and thus, through the continuity of the normal velocity, provides both the amplitude and phase of the structural vibration of the source. Through a series of ...