SOUND GENERATION IN THE VICINITY OF THE SEA-SURFACE - SOURCE MECHANISMS AND THE COUPLING TO THE RECEIVED SOUND FIELD

This paper presents a theory of the mechanisms of sound generation in fluids containing moving fluid interfaces, i.e., discontinuities in density and sound speed, following the approach of Lighthill [Proc. R. Soc. London Ser. A 211, 564-587 (1952)]. It is shown that in addition to the expected volume distributions of quadrupoles, motion of the discontinuities radiate sound equivalent to that from distributed monopoles and dipoles in place of the surfaces. These sources account both for the generation of sound and for the effect of the surfaces on the radiation of that sound. The results are applied to sound generation in the vicinity of the sea surface. All sources are inherent in the theory. The second part of this paper derives a direct relationship between the sound pressure spectrum in the ocean and the frequency wave-number spectra of the near surface fluid processes responsible for generating the sound, on the assumption that these fluid processes are statistically homogeneous in the horizontal plane. The effects of refraction and bottom reflections are ignored. The results are given in terms of "coupling factors" that are measures of the extent to which the source field couples to the received sound field. The coupling factors are evaluated and the results give some insight into the source characteristics required to ensonify the far field.