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A hydrophone Not to be confused with hydraulophone, a musical instrument. A (Greek "hydro" = "water" and "phone" = "sound") is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change. Such piezoelectric materials, or transducers can convert a sound signal into an electrical signal since sound is a pressure wave. Some transducers can also serve as a projector (emitter), but not all have this capability, and may be destroyed if used in such a manner. A hydrophone can "listen" to sound in air, but will be less sensitive due to its design as having a good acoustic impedance match to water, the denser fluid. Likewise, a microphone can be buried in the ground, or immersed in water if it is put in a waterproof container, but will give similarly poor performance due to the similarly bad acoustic impedance match. Contents 1 History 2 Directional hydrophones 2.1 Focused Transducers 2.2 Arrays 3 See also 4 Notes 5 References 6 Links History The hydrophone was used late in World War I. Convoy escorts used them to detect U-boats, greatly lessening the effectiveness of the submarine. Ernest Rutherford, in England, led pioneer research in hydrophones using piezoelectric devices. His only patent was for a hydrophone device. From late in World War I until the introduction of active sonar, hydrophones were the sole method for submarines to detect targets while submerged, and remain useful today. Directional hydrophones A small single cylindrical ceramic transducer can achieve near perfect omnidirectional reception. Directional hydrophones increase sensitivity from one direction using two basic techniques: Focused Transducers This device uses a single transducer element with a dish or conical-shaped sound reflector to focus the signals, in a similar manner to a reflecting telescope. This type of hydrophone can be produced from a low-cost omnidirectional type, but must be used while stationary, as the reflector impedes its movement through water. A new way to direct is to use a spherical body around the hydrophone. The advantage of Directivity Spheres is that you can move the hydrophone within the water and you get rid of the interferences produced by a conical-shaped element. Arrays Multiple hydrophones can be arranged in an array so that it will add the signals from the desired direction while subtracting signals from other directions. The array may be steered using a beamformer. Most commonly, hydrophones are arranged in a "line array"[citation needed] but may be in two or three dimensional arrangements. 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 submarines during the Cold War along a line from Greenland, Iceland and the United Kingdom known as the GIUK gap.[1] See also Communication with submarines Underwater acoustics Sonar Reflection seismology Notes ^ Mackay, D.G. "Scotland the Brave? US Strategic Policy in Scotland 1953-1974." Glasgow University, Masters Thesis (research). 2008. Accessed 12 October 2009. References Pike, John (1999). SOSUS Retrieved Jan. 28, 2005. Watlington, Frank (1979). How to build & use low-cost hydrophones. (ISBN 0830610790) Unknown. hydrophone Retrieved Jan. 28, 2005. Unknown. (2005) Schlumberger Oilfield Glossary: Term 'hydrophone' Retrieved Jan. 28, 2005. Links Wikimedia Commons has media related to: Hydrophone High Sensitivity PiezoComposite Hydrophones Sonar Surround Hydrophones Free underwater surround recordings. Hydrophones and accessories Directivity Sphere for hydrophones DOSITS Hydrophone introduction at Discovery of Sound in the Sea Live hydrophone streams from killer whale habitat RESON Hydrophones High Quality Hydrophones Passive Acoustic Monitoring Using hydrophones to monitor underwater sounds Build your own hydrophone (free instructions) Build your own hydrophone #2 Online tutorial at The Freesound Project. Hydrophone recordings at The Freesound Project. Precision Acoustics useful resource on hydrophones The British Library Sound Archive contains many wildlife and atmospheric recordings made using hydrophones.