Mechanisms of Sound Generation at the Ocean Surface

Abstract

Fluid motions at the ocean surface provide a wide variety of mechanisms that might be the cause of underwater sound. This paper is devoted to help understand better these mechanisms, and to help identify those sources most responsible for the received noise in the ocean. Flows at the sea surface, as well as the sound they are generating, are governed by equations of mass and momentum conservation. They can be cast into an acoustic analogy which interprets the sound generation by regarding some terms in the conservation laws as sources that drive the sound field. Though the equations describing the conservation laws are precise, the choice of the`source? terms is not at all unique; what should, or should not, be regarded as sources of sound is largely a point of view. The arbitrariness of choosing the source terms for an acoustic analogy in aero- and hydro-acoustics is a reflection of the well-known ambiguity that an infinite number of different source descriptions can produce an identical wave field. However, for the problem of underwater sound generation, as indeed for most other flow noise problems, we only have a very crude knowledge of the source flow, so that it is crucially important, in casting an acoustic analogy from the exact equations of motion, to specify the source terms as precisely as possible in order to reveal correctly the dominant sound-producing processes. In doing so, the effects of the ocean surface that separates the air and water must be taken into account with great care; this density interface plays a key role in the problem. Due to its presence, powerful acoustic sources may be induced, while equally important, sources which are efficient in other situations may become negligible. We start with an account of the basic principles of flow noise generation, with particular reference to the ocean sound problem. This enables us to examine a selection of flow processes which are commonly believed to be most likely responsible for the surface motion associated oceanic noise. We will demonstrate that breaking surface waves in the ocean provide the dominant sources of underwater sound by causing rapid momentum variations at the sea surface. These dominant sources arise directly from the splashing of water sprays, and are much more efficient than other processes; entrainment of air bubbles and their subsequent unsteady motions in water are negligible because they suffer from fatal destructive interference from their negative images at the sea surface. The interactions of surface waves may also be irrelevant to the ocean sound problem because they can only provide very weak acoustic sources, in comparison, for most of frequencies of interest, with the sources from breaking waves, and for very low frequencies (below 10 Hz, say), with those induced directly by the turbulent airflow.