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The SOFAR channel (short for Sound Fixing and Ranging channel), or deep sound channel (DSC), is a horizontal layer of water in the ocean at which depth the speed of sound is at its minimum. The SOFAR channel acts as a waveguide for sound, and low frequency sound waves within the channel may travel thousands of miles before dissipating. This phenomenon is an important factor in submarine warfare. The deep sound channel was discovered and described independently by Maurice Ewing, Stanley Wong and Leonid Brekhovskikh in the 1940s.
The SOFAR channel is centred on the depth where the cumulative effect of temperature and water pressure (and, to a lesser extent, salinity) combine to create the region of minimum sound speed in the water column. Pressure in the ocean increases linearly with depth, but temperature is more variable, generally falling rapidly in the main thermocline from the surface to around a thousand metres deep, then remaining almost unchanged from there to the ocean floor in the deep sea. Near the surface, the rapidly falling temperature causes a decrease in sound speed, or a negative sound speed gradient. With increasing depth, the increasing pressure causes an increase in sound speed, or a positive sound speed gradient. The depth where the sound speed is at a minimum is called the sound channel axis.
Near Bermuda, the sound channel axis occurs at a depth of around 1000 metres. In temperate waters, the axis is shallower, and at high latitudes (above about 60°N or below 60°S) it reaches the surface.
Sound propagates in the channel by refraction of sound, which makes sound travel near the depth of slowest speed. If a sound wave propagates away from this horizontal channel, the part of the wave furthest from the channel axis travels faster, so the wave turns back toward the channel axis. As a result, the sound waves trace a path that oscillates across the SOFAR channel axis. This principle is similar to long distance transmission of light in an optical fibre.
Mysterious low-frequency sounds, attributed to fin whales (Balaenoptera physalus), are a common occurrence in the channel. Scientists believe fin whales may dive down to this channel and "sing" to communicate with other fin whales many kilometers away.
During World War II, Dr. Maurice Ewing suggested that dropping into the ocean a small metal sphere (called a SOFAR bomb or SOFAR disc), specifically designed to implode at the SOFAR channel, could be used as a secret distress signal by downed pilots.
The novel The Hunt for Red October describes the use of the SOFAR channel in submarine detection.
- Ocean acoustic tomography: A technique to measure ocean temperatures and currents by the time delay of sounds between two distant instruments
- Search for Malaysia Airlines Flight 370: Sounds carried by the SOFAR channel were analyzed to determine if they detected a possible ocean impact of a passenger jet which disappeared in the Southern Indian Ocean
- SOSUS: Hydrophone system to detect submarine movements during Cold War
- Navy Supplement to the DOD Dictionary of Military and Associated Terms (PDF). Department Of The Navy. August 2006. NTRP 1-02.
- "The Heard Island Feasibility Test" (PDF). Acoustical Society of America. 1994.
- Citation for Leonid Maximovich Brekhovskikh ...for pioneering contributions to wave propagation and scattering
- Orientation by Means of Long Range Acoustic Signaling in Baleen Whales, R. Payne, D. Webb, in Annals NY Acad. Sci., 188: 110–41 (1971)
- "Sound Channel, SOFAR, and SOSUS". Robert A. Muller. Retrieved 2007-04-14.
- The SOFAR or deep sound channel, from NOAA
- A sound pipeline, from the National Academy of Sciences
- SOSUS, the "Secret Weapon" of Underwater Surveillance by Edward C. Whitman. Undersea Warfare
- Richard Muller, UC Berkeley – lecture on waves, SOFAR and the Roswell UFO Incident