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A hydrophone is a submersible sound sensor. It is analogous to a microphone or an ear for listening to underwater sound. At its core is a piezoelectric material 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 in fluids. 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.

Contents 1 History 2 Directional hydrophones 2.1 Focused Transducers 2.2 Arrays 3 Relationship between hydrophone, geophone, and microphone 3.1 Geophysics 4 See also 5 References 6 Links

The hydrophone was used late in World War I. American convoys used them to detect German U-boats, greatly lessening the effectivity of the submarine.^{[citation needed]} Rutherford, in England, lead pioneer research in hydrophones using piezoelectric devices. His only patent was for a hydrophone device.

A small single cylindrical ceramic transducer can achieve near perfect omnidirectional reception. Directional hydrophones increase sensitivity from one direction using two basic techniques:

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.

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.

Transducer:	Geophone	Hydrophone	Microphone/Loudspeaker	Ionophone	Informatic transducer
State of matter:	Solid	Liquid	Gas	Plasma	Ideas/Informatics
Classical Element:	Earth	Water	Air	Fire	Idea
Greek:	Geo/Gaia	Hydro/Hydor	Aero	Pyro/Ion(ἰόν)
Latin:					Quintessence (fifth-element)

Whereas the hydrophone in water is analogous to a microphone in air, the geophone is a form of listening device ideally suited for being in earth (solid-matter).

Thus, depending on the state of matter of the medium in which the listening device is immersed, we have three similar (analogous) listening devices:

• Solid ("Earth"); listening devices are called Geophones ("Geo" means Earth);
• Liquid ("Water"); listening devices are called Hydrophones ("Hydro" means Water);
• Gas ("Air"); listening devices are called microphones.

Note that the term "Aerophone" which is the etymologically correct term for microphone, is already used as a musical instrument that makes sound from air.

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 more dense fluid. Likewise, a microphone can be buried in the ground, or immersed in water if is put in a waterproof container, but will give similarly poor performance due to the similarly bad acoustic impedance match.

The term "hydrophone" is of Greek origin, denoting "hydro" = "water" and "phone" = "sound" (in Greek language).

The three corresponding terms for musical instruments that make sound in each of the three states-of-matter are as follows:

• Γαια, gaia earth; Solid: Gaiaphone^[1];
• ύδωρ, hydor, water; Liquid: Hydraulophone;
• άήρ, aer, air; Gas: Aerophone.

Geophones are a low frequency form of listening device, analogous to a hydrophone, used by geologists and geophysicists in detecting seismic energy. They are combined to form streamers that are towed by seismic vessels or deployed in a borehole.

• underwater acoustics
• sonar

• 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.

• DOSITS Hydrophone introduction at Discovery of Sound in the Sea
• orcasound.net Live hydrophone streams from killer whale habitat
• Passive Acoustic Monitoring Using hydrophones to monitor underwater sounds
• Build your own hydrophone (free instructions)
• Precision Acoustics useful resource on hydrophones