Infrasonic ambient noise in the ocean due to atmospheric pressure fluctuations on the surface

Abstract

The theory of infrasonic ambient noise in a semiinfinite, isotropic ocean due to random wind-pressure fluctuations acting on the sea surface is reexamined. The surface pressure field is modeled as a Poisson distribution of point forces. This provides a boundary condition that must be satisfied by the solution of the homogeneous Helmholtz equation for the resultant acoustic field in the ocean. Using this solution the spatial coherence function of the ambient noise is constructed, from which the directionality in the vertical is derived and shown to be the same as that for the noise field produced by a random distribution of vertical dipoles on the surface. The power spectrum of the noise scales with the point spectrum of the wind-pressure fluctuations, which is determined empirically from published measurements of wind-pressure spectra taken over water, and with the square of the Helmholtz number for the turbulent sources. Since the Helmholtz number is very much less than unity, it is concluded that wind turbulence is an inefficient mechanism for producing sound in the ocean. A comparison of the final result for the ambient noise power spectrum with measured noise spectra, taken by several investigators in various parts of the world, indicates that wind turbulence generally makes an insignificant contribution to the infrasonic ambient noise in the ocean.