Mid- to high-frequency ambient noise anisotropy and notch-filling mechanisms

Abstract

Mid- to high-frequency noise (>3 kHz) is dominated by sub-surface bubble breaking caused by wind-driven wave action. For a submerged receiver in a downward-refracting environment, a noise notch centered about the horizontal is predicted in the vertical noise directionality. The noise notch width is related to the ratio of the sound speed at the receiver to the sound speed at the surface. The noise notch depth is determined by other noise filling mechanisms. For receiver arrays with vertical beamwidths sufficiently narrow to look in the notch, the characterization of the notch level is important for the prediction of beam noise levels near the horizontal. Three notch-filling mechanisms are described: down-slope conversion of surface energy due to bathymetry, volume scattering from volume inhomogeneities, and internal waves. A ray-based noise model is used to examine these mechanisms, as well as the sensitivity to uncertainty in environmental inputs.