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The 'SOFAR channel' (sound fixing and ranging channel), or 'deep sound channel' (DSC), Navy Supplement to the DOD Dictionary of Military and Associated Terms, , , , Department Of The Navy, 2006, NTRP 1-02 is a region of water deep in the ocean centered on where the speed of sound is at a minimum. The SOFAR channel acts as a waveguide for acoustics, and low frequency sound waves within the channel may travel thousands of miles before dissipating.^[1] This phenomenon is an important factor in submarine warfare. The deep sound channel was discovered and described independently by Dr. Maurice Ewing, and Leonid Brekhovskikh in the 1940s.[1]

The SOFAR channel is centered on the depth where the cumulative effect of temperature and water pressure (and, to a smaller 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 meters 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 meters. 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 fiber.
Mysterious low-frequency sounds, attributed to humpback whales and other baleen whales, are a common occurrence in the channel. Scientists believe humpback whales may dive down to this channel and "sing" to communicate with other humpback whales many kilometers away.
During World War II, Dr. Maurice Ewing suggested that dropping a small metal sphere into the ocean, specifically designed to implode at the SOFAR channel, could be used as a secret distress signal by downed pilots.^[2]
The novel ''The Hunt for Red October'' describes the use of the SOFAR channel in submarine detection.
The conjectured existence of a similar channel in the upper atmosphere, theorized by Dr. Ewing, led to Project Mogul, carried out from 1947 until late 1948.

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References

See also

External links

References

1. The Heard Island Feasibility Test
2.
Sound Channel, SOFAR, and SOSUS

See also

★ Bathythermograph (BT)

★ Thermocline

★ SOSUS

★ Underwater acoustics

★ Ocean acoustic tomography

External links

★ 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''