The secondary field of a parametric source following interaction with sea surface

The effects of a flat sea surface on the secondary wave field fora nonsatur ation-limited parametric source with a piston-type transmit transducer have been assessed both theoretically and experimentally. When the virtual arra y of the parametric source interacts with the sea surface, the amplitude of the secondary signal is reduced. In addition to the normal interference be tween the direct and surface reflection paths, there is also one extra mech anism which causes the reduction of the secondary signal level. This is the destructive summation of the secondary field generated before and after in tersection with the water surface. A numerical model and the Westervelt mod el are used to predict the secondary field which is a function of grazing a ngle. Experiments in an in-door laboratory, tank have been carried out to m easure the primary and secondary fields of a parametric array under these c ircumstances. It is shown that the surface reflection affects the beam patt ern of the secondary field and the induced reduction in the signal level de pends on the characteristics of the parametric source and the geometry of t he problem. The level of reduction caused by the effects of the surface int eraction approaches a constant at large grazing angles. (C) 1999 Acoustical Society of America. [S0001-4966(99)00206-4].