Abstract:

Underwater measurements of the radiated sound power from a vibrating structure are conventionally performed either by direct measurements in a large body of water or by reverberation measurements in a water tank. Both methods have inherent drawbacks. Direct measurements in a large body of water require a large scale operation and are relatively expensive. Reverberation measurements are only accurate at high modal densities, and thus a very large tank is required to obtain accurate low-frequency predictions. To avoid both difficulties, a hybrid approach has been devised. Experimental measurements of the vibrations of the structure are taken in a water tank using a scanning laser Doppler vibrometer, where the laser beam is shone onto the submerged structure through an optical quality window. The surface vibration data are then used to derive the specified normal velocity for a numerical calculation of the radiated sound power radiated in free-field conditions. The laser allows dense surface meshes to be defined such that the calculations can be performed to relatively high frequencies. To demonstrate the general accuracy of the calculations, radiation efficiencies are computed for submerged unbaffled plates, and the results are compared to reverberation measurements and to fully numerical predictions.