The mixing effect of the wind on the near-surface water of the sea produces a layer of almost constant temperature and salinity. This so-called 'mixed layer' is a regular feature of the world's oceans. Its thickness, determined largely by the wind, varies with time and position (in temperate latitudes the median mixed layer thickness varies from about 60 m in winter to about 20 m in summer). Within a totally mixed layer the speed of sound increases linearly with depth at a rate of approximately 1.7 m/s every 100 m due to the increasing pressure. This feature causes sound to be refracted upwards, thereby trapping a proportion of the energy emitted by a source in the layer. On account of this 'ducting' property the mixed layer is commonly called the 'surface duct' by acousticians. Good transmission as a result of surface ducting was recognized as early as 1937 and continues to generate interest in underwater acoustics. Below the mixed layer the temperature decreases rapidly with depth (the thermocline layer) causing strong negative gradients in the vertical sound speed profile. The simplest category of surface duct, then, comprises a layer of linearly increasing sound speed, the upper boundary formed by the sea surface and the lower boundary by the strong negative thermocline gradient.
Underwater sound propagation in surface ducts
Die Ausbreitung vom Unterwasserschall entlang der Meeresoberflaeche
1985
7 Seiten, 2 Bilder, 14 Quellen
Conference paper
English
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