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Sonar and the Not So Silent Seas

May 19, 2017

Snorkeling over a coral or oyster reef, you can often hear the cracking/popping sound produced by a snapping (pistol) shrimp. Barely 1 to 2 inches in length, it can produce a sound of up to 220 decibels, the equivalent of the Battleship New Jersey firing all 9 of its sixteen-inch guns! The shrimp uses the sound to stun or kill its prey. A colony of these tiny crustaceans are said to be capable of interfering with sonar and underwater communications.
The oceans are not a Silent World as might be expected from the title of Jacques-Yves Cousteau book. The loudest recorded underwater noise was from an “icequake,” the cracking of Antarctic ice. The calls of a blue whale had been detected from a distance of some 2,300 miles. On average, the speed of sound in salt water is 3,490 mph versus its speed in air of 760 mph.
One of the noisiest creatures in Long Island Sound is the male oyster toadfish. The male establishes a nest and begins to call for a mate with a foghorn-like sound (110 decibels -equivalent of a power saw). After seeing the toadfish close-up underwater, one can safely say that it’s definitely not his looks that attracts the “girl.” The call instead must be music to her “ears.”  Next in line of our common noise makers has to be our two species of bait-stealing sea robins. Variously described a squawks or barks, the fish uses its calls in mating and establishing territory.  
Along the East Coast, there are over 150 species of fish that can contribute to underwater sounds. That even includes the Northern seahorse and its close relative the pipefish that produce a snapping sound. The good-tasting blue mussels also make a sound as they changed position for protection or capturing food. They do so by releasing and reattaching their anchoring (byssal) threads.
From late November until April, harbor seals migrate into the Sound. They produced underwater sounds during mating season. On the surface, they make a variety of sounds that include barks, grunts and calls to  their pups. Among our most rare subsurface noise makers are the minke and beluga whales. In 2015, a humpback whale was even sighted off Stamford, CT.
In the 1820s, using an underwater bell hung from one boat and an underwater ear-trumpet from another, a Swiss physicist and a French mathematician, were the first to measure the speed of sound in water. During the late 1800s, underwater signal bells were installed on three coastal lightships. Their sound, could be detected by properly equipped vessels, from a distance of up to 12 miles. The bells were especially useful in detecting the location of the navigational aid in limited visibility. But it wasn’t until the Titanic’s sinking by an iceberg, that a great deal of more attention was directed toward detecting underwater hazards, with devices other than a weighed line casted over the side of ships.
Soon after the marine disaster, a Canadian designed a device that produced and received a signal capable of echo-locating icebergs at a distance of nearly 2 miles. But submarine warfare of World War I (1912-1918) motivated the navies of Britain and the U.S. to carry out further research (during the war, “subs sank 4,800 merchant ships”). Though the navies developed a hydrophone which was towed behind a ship, it would not be until the near the very end of the war that more effective echo-sounding sonar systems were produced.
The first SONARs (the acronym for SOnar Navigation And Ranging -coined in 1942) were passive listening devices; there was no transmitted signal. To detect a submarine, the sonarman used two air-tube earphones (similar to a doctor’s stethoscope) connected to an underwater sound receiver. With the device, the clatter and direction of an engine or propeller could be detected. The operator however, had to learn to differentiate the sounds from the multitude of marine life noises.
From 1925 through 1927, German oceanographers conducted the first underwater topographic survey using echo-sounding sonar. During the operation, they mapped part of the Mid-Atlantic Ridge, an underwater mountain range that rises nearly 2 miles above the ocean floor. However, it was just the beginning and according to ocean explorer Robert Ballard, and even today “we know less about 71 percent of the Earth’s landscape (underwater) than the far side of the Moon.”
The beginning of World War ll was further motivation for improving equipment to detect submarines. Even before entering the war, U. S. researchers coordinated efforts with the British Technical and Scientific Mission in developing sonar. During the last years of the war an acoustic transducer that converted electrical energy to sound waves was developed, bringing about rapid developments.
Towed-array sonars are one of today’s generation of naval equipment. They are employed to “detect, track and classify quiet, modern submarines in open and coastal waters.”
Fishing sonars have also come along way with the introduction of CHIRP sounders (Compressed High Intensity Radiated Pulse). They greatly improve imaging and resolution of fish and underwater structures.
But there is a downside to naval sonar and other equipment that add to man-made marine noise; they have been implicated in mass strandings and deaths of whales. Offshore explorations for oil and gas have employed enormous blasts of compressed air, set off “every 12 seconds for weeks to month at a time.” The blasts interfere with whale calls and scatter schools of commercial fish to a point that companies have been sued for compensation. The oil industry has been restricted from exploring certain areas, and alternatives are being developed for air-guns. The International Maritime Organization has also established ship quitting guidelines for commercial vessels.
The Navy has restricted sonar activities from certain critical marine habitats(especially during mating and migrating seasons). However, its ability to remain vigilant and continue to improve its detection of potential enemy submarine remains paramount, even in times of peace.

 

 

 

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