Sources of Ambient Noise in the Ocean: An Experimental Investigation

摘要

The general characteristics of underwater sound produced at the ocean surface have been known for many years and recent measurements have also described the sound of rainfall. The mechanisms which produce these sounds have remained a mystery. This report describes a series of laboratory experiments in which various simple mechanisms in the 0.5-100 kHz frequency range were examined. A large part of the work describes the sounds made by the impact of a drop of water on the water surface. It is found that two types of sound are emitted: first, a sharp spike radiated when the drop first strikes the surface and second, a damped sinewave emitted when a bubble is entrained below the water surface. For a certain range of drop sizes and velocities a bubble will always be entrained; this phenomenon has been named regular entrainment. The hydrodynamics of a drop impact are discussed in an attempt to show why regular entrainment occurs; a qualitative explanation is described using computer modelling. The sound of rainfall on water is studied in detail; an important feature of the acoustic spectrum is a peak at about 14 kHz. It is shown that this peak is caused by regular entrainment and not by initial impacts as one author has suggested. Experimental results enable us to predict the spectrum levels which rain of a given intensity would produce; the predictions compare reasonably well with real rain data. Other experiments examined the sound of a breaking wave and of bubbles interacting with a submerged jet of water. Theses. (JHD)