Product Code Database
A hydrophone () is a microphone designed for underwater use, for recording or listening to underwater sound. Most hydrophones contain a Piezoelectricity transducer that generates an electric potential when subjected to a pressure change, such as a sound wave.
A hydrophone can also detect airborne sounds but is insensitive of them because it is designed to match the acoustic impedance of water, a denser fluid than air. Sound travels 4.3 times faster in water than in air, and a sound wave in water exerts a pressure 60 times more than what is exerted by a wave of the same amplitude in air. Similarly, a standard microphone can be buried in the ground, or immersed in water if it is put in a waterproof container but will give poor performance because of the similarly-bad acoustic impedance match.
The first submarine to be detected and sunk using a primitive hydrophone was the German submarine UC-3 on 23 April 1916. UC-3 was detected by the anti-submarine trawler Cheerio as the Cheerio was directly over the UC-3; the UC-3 was then caught in a steel net dragged by the trawler, and sank after a large underwater explosion.
Later in the war, the British Admiralty belatedly convened a scientific panel to advise on how to combat U-boats. It included the Australian physicist William Henry Bragg and the New Zealand physicist Sir Ernest Rutherford. They concluded that the best hope was to use hydrophones to listen for submarines. Rutherford's research produced his sole patent for a hydrophone. Bragg took the lead in July 1916 and he moved to the Admiralty hydrophone research establishment at Hawkcraig on the Firth of Forth.Wood 1930, p. 457.
The scientists set two goals: to develop a hydrophone that could hear a submarine despite the noise generated by the patrol ship carrying the hydrophone, and to develop a hydrophone that could reveal the bearing of the submarine. A bidirectional hydrophone was invented at East London College. They mounted a microphone on each side of a diaphragm in a cylindrical case; when the sounds heard from both microphones have the same intensity, the microphone is in line with the sound source.Wood 1930, p. 457.
Bragg's laboratory made such a hydrophone directional by mounting a baffle in front of one side of the diaphragm. It took months to discover that effective baffles must contain a layer of air.Van der Kloot 2014, p. 110. In 1918, airships of the Royal Naval Air Service engaged in anti-submarine warfare experimented by trailing dipped hydrophones.Report AIR 1/645/17/122/304 – National Archives Kew. Airship Hydrophone experiments. Bragg tested a hydrophone from a captured German U-boat and found it inferior to British models. By the end of the war, the British had 38 hydrophone officers and 200 qualified listeners, paid an additional 4 d per day.Van der Kloot 2014, p. 125.
From late in World War I until the introduction of active sonar in the early 1920s, hydrophones were the sole method for submarines to detect targets while submerged; they remain useful today.
SOSUS hydrophones, laid on the seabed and connected by underwater cables, were used, beginning in the 1950s, by the U.S. Navy to track movement of Soviet Union submarines during the Cold War along a line from Greenland, Iceland and the United Kingdom known as the GIUK gap.Mackay, D.G. " Scotland the Brave? US Strategic Policy in Scotland 1953–1974". Glasgow University, Masters Thesis (research). 2008. Accessed 12 October 2009. These are capable of clearly recording extremely low frequency infrasound, including many unexplained ocean sounds.
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