The undersea sound pipe

The speed of sound waves travelling in seas and oceans varies with depth due to changes in temperature and density under the water’s surface. The speed of sound initially falls with depth as temperature drops beneath warm surface waters, but then rises again as pressure increases at greater depth. This speed distribution creates a sound duct, known as the sound fixing and ranging (SOFAR) channel or deep sound channel.

The SOFAR channel is found at a depth that rises from 1000 m at mid-latitudes, to close to the ocean’s surface in polar regions. Much like a fibre optic cable guides light rays, so the varying refractive index of sea water refracts sound waves along the channel’s axis. The channel enables blue whales to communicate over long distances. Researchers have found that the sound of small, underwater explosions can travel thousands of kilometres in the SOFAR channel.