Effects of water surface on the field of a parametric source

The effects of a flat sea surface on the secondary wave field for a parametric acoustic source with a piston type transmit transducer have been assessed both theoretically and experimentally. When the virtual array of a parametric source is truncated by the sea surface, the amplitude of the secondary signal is reduced. In addition to the normal cancellation between the direct and surface reflection paths, there are two more mechanisms which cause the reduction of the signal level with a rough sea surface. The first one is the destructive summation of the secondary field before and after the intersection with the surface, the second one is the loss of the coherence of the primary signals after reflection. The Westervelt model is used to predict the secondary field. Experiments in an indoor laboratory tank have been carried out to measure the primary and secondary fields. It is found that, with a flat water surface, the reduction in the signal level depends on the characteristics of the parametric source and the geometry of the problem. Both the theory and experiments show that there is a maximum loss caused by the truncation. The experimental results indicate that the maximum loss is almost 10 dB in signal level for the problem concerned, while the theory predicts an even higher value.