Horizontal sound paths in the SOFAR channel (top) and near the surface if the surface layer is well mixed (bottom). The diagram at the left gives the vertical sound velocity profile for each case; note the very different vertical scales. The red circle indicates the sound source.

The principle of sound propagation is that sound rays always bend towards the region of lower sound speed. This produces a sound channel near 1000 m depth (top) and a shadow zone below the surface mixed layer (bottom) to which sound from the source cannot penetrate. Since sound rays are reversible, this also means that sound produced in the shadow zone cannot be heard by a sound detector placed at the location of the red circle.

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