The internal waves effect on the noise vertical directionality in shallow water

Acoustic environment is known to influence the vertical directionality of the ambient noise in shallow water. For example, in shallow water with a downward refractive (summer) sound speed profile, the noise vertical directionality is expected to show a deep (>10 dB) null in the horizontal direction for wind generated noise below 1 kHz. This ''horizontal null'' is a consequence of the fact that low order modes are weakly excited by the noise sources which are located near the ocean surface. This phenomenon has been experimentally observed and is not expected to occur in a upward refractive (winter) sound speed profile environment. However, when internal waves are present, the deep null may disappear or significantly weaken due to the ''strong'' mode coupling effect caused by the scattering of the noise field from the internal waves. In the internal wave fields, the conversion of higher order modes (generated by the surface noise sources) to low order modes by the internal waves can fill in the deep (> 10 dB) null which would otherwise be expected. We present numerical calculations of the vertical noise directionality at frequencies below 1 kHz for typical shallow water environments and investigate the potential effects of internal waves on the ambient noise directionality. The numerical simulations are compared with ambient noise directionality recently measured in an area south of Long Island which showed the presence and absence of the ''null'' at different times. The fill-in of the null can be explained by noise from distant ships when internal waves are present.