The effect of sea-surface roughness on ambient noise vertical directionality in shallow water waveguide

The vertical directionality of wind-generated noise in the vertical direction is influenced by the sea state, which in turn is dependent on the acoustic source distribution associated with breaking surface waves and by the acoustic propagation environment. At present, many ocean ambient noise vertical directionality models are available, such as K/I model, CAREY model, CANARY model. And most ambient noise models treat the sea surface as a flat pressure-release boundary, and the effects of the sea surface and bottom roughness are not considered in these models. In shallow water, both the bottom and surface roughness (sea state) can be factors which influence the directionality of ambient noise. The aim of this paper is to assess the effect of sea surface roughness on the accuracy of the vertical directionality of noise. Simulations were performed using a normal-mode model RDM developed by Westwood, which can accurately evaluate the branch line integral by a number of separate modes. Both the “over head noise sources” contribution and the “distant noise sources” contribution can be computed by the model. The application of the RDM model results in a magnitude reduction in computation time compared with existing noise models. The surface scattering phenomenon was introduced by using modal attenuation coefficients based on the Kirchhoff approximation theory. Introducing the attenuation into the noise model caused by the rough surface, and influences of the surface roughness on the vertical directionality of the ambient noise is investigated. Several examples are given with respect to the environment, with different roughness surface and constant sound-speed in water. The results show that surface roughness can alter the noise power distribution in almost all grazing directions, especially for moderate and high grazing angles, because the higher order normal modes suffer more energy loss due to the surface roughness. The work reveals the importance of rough interface scattering in forecasting vertical directionality of ambient noise.