Acoustic noise generation at the air/ocean boundary

The downwash from a hovering aircraft generates acoustic sources at the ocean surface through the same mechanisms as wind generated ambient noise-wave breaking and spray generated radially pulsing bubbles which in turn act as dipole sources because of the pressure release ocean surface. The empirically known dependence of wind generated ambient noise on the wind velocity is used to give the acoustic source strength as a function of the downwash velocity on the ocean surface. The velocity field of the rotor downwash can be deduced through Froude-Rankine actuator disc theory combined with a cylindrical vortex model to account for the ground effect. By integrating the resulting (variable) source strength over the ocean surface, quantities such as the noise level, the received intensity per unit solid angle, and the correlation functions are completely determined.