Sound Generation in the Ocean Under a Turbulent Airflow

Abstract

A general theory is developed, based on the extension of the Lighthill-Curle theory of aerodynamic sound by Ffowcs Williams and Hawkings, to account for various features in the problem of sound generation in the ocean. Some particular situations are examined in detail. It is shown that, whenever the ocean surface is non-linearly deformed, any quadrupole radiations become negligible; the most significant sound arises from surface motions. This surface source sound is dominant over the aerial quadrupole radiation by the inverse of the characteristic Mach number in the air and bigger than any quadrupole field due to non-linear motions in the water body by the ratio of a typical surface wavelength to a typical acoustic wavelength. A simple relation is obtained to relate the frequency spectrum of the sound pressure to the surface wave spectrum, which predicts the underwater ambient sound in good agreement with meassurements. The theory is also applied to examine the sound generation mechanism advocated by Brekhovskikh. It is shown that, theoretically, the sound calculated according to Brekhovskikh’s theory overwhelms the aerial turbulence radiation when the gravity surface waves have wavelengths much smaller than the height of the turbulence source layer. However, in practical applications, this theory underestimates the sound level and numerical calculations show that this mechanism is less important in the production of underwater sound than that of fully non-linear surface motions. Finally, an expression is derived to include the effects of wave foamings and rainstorms which introduce a powerful dipole distribution on the ocean surface.