View source for Hydrophone

{{Short description|Underwater microphone}}
{{Distinguish|text=[[hydraulophone]], a musical instrument}}
{{Use dmy dates|date=October 2020}}

A '''hydrophone''' ({{lang-grc|ὕδωρ + φωνή||water + 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 an electric potential when subjected to a pressure change, such as a sound wave.

A hydrophone can detect airborne sounds, but will be insensitive 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 that 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 due to the similarly bad acoustic impedance match.

==History==

[[File:Hydrophone being lowered into the Atlantic.jpg|thumb|A hydrophone being lowered into the North Atlantic]]
The first hydrophones consisted of a tube with a thin membrane covering the submerged end and the observer's ear on the other end.{{hsp}}<ref>{{cite book |last1=Wood |first1=A. B. |title=A textbook of sound |date=1930 |publisher=G. Bell and Sons |location=London |pages=446–461}}</ref> The design of effective hydrophones must take into account the acoustic resistance of water, which is 3750 times that of air; hence the pressure exerted by a wave of the same intensity in air is increased by a factor of 3750 in water.     The American Submarine Signaling Company developed a hydrophone to detect underwater bells rung from lighthouses and lightships.{{hsp}}<ref>{{cite book|last1=Van der Kloot|first1=William|title=Great Scientists wage the Great War|date=2014|publisher=Fonthill|location=Stroud|page=104}}</ref> The case was a thick, hollow brass disc {{convert|35|cm|inch}} in diameter.  On one face was a  {{convert|1|mm|inch}} thick brass diaphragm which was coupled by a short brass rod to a [[carbon microphone]].

===World War I===

Early in the war, French President [[Raymond Poincaré]] provided [[Paul Langevin]] with the facilities needed to work on a method to locate submarines by the echos from sound pulses. They developed a [[piezoelectric]] hydrophone by increasing the power of the signal with a [[vacuum tube]] amplifier; the high [[acoustic impedance]] of piezoelectric materials facilitated their use as underwater transducers. The same piezoelectric plate could be vibrated by an electrical oscillator to produce the sound pulses.{{hsp}}<ref>Van der Kloot, 2014, pp. 110-112.</ref>

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.<ref name="Thomas">{{cite news |last1=Thomas |first1=Lowell |title=Fighting the Submarine |url=https://books.google.com/books?id=wt4DAAAAMBAJ&pg=RA1-PA24 |work=Popular Mechanics |date=July 1929}}</ref><ref name="Brodie">{{cite book |last1=Brodie |first1=Bernard |last2=Brodie |first2=Fawn M. |title=From Crossbow to H-bomb: the evolution of tactics and warfare |date=1973 |publisher=Indiana University Press |isbn=0253201616 |page=184 |edition=First Midland |url=https://books.google.com/books?id=mAsrwMrfaoQC&pg=PA184 }}</ref>

[[File:Hydrophone drawing.jpg|thumb|Hydrophones and directional hydrophones using a baffle.]]

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]].{{hsp}}<ref>Wood 1930, p. 457.</ref>

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.{{hsp}}<ref>Wood 1930, p. 457.</ref>

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.{{hsp}}<ref>Van der Kloot 2014, p. 110.</ref> In 1918, airships of the Royal Naval Air Service engaged in [[anti-submarine warfare]] experimented by trailing dipped hydrophones.<ref>Report AIR 1/645/17/122/304 - National Archives Kew.  Airship Hydrophone experiments.</ref> 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.{{hsp}}<ref>Van der Kloot 2014, p. 125.</ref>

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.

==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 the hydrophone can be moved within the water, ridding it of the interferences produced by a conical-shaped element.

===Arrays===
Multiple hydrophones can be arranged in an [[microphone array|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 [[beamforming|beamformer]].  Most commonly, hydrophones are arranged in a "line array"<ref>{{Cite book|url=https://books.google.com/books?id=azuIDwAAQBAJ&q=hydrophones+are+arranged+in+a+%22line+array%22&pg=PA39|title=Underwater Acoustic Signal Processing: Modeling, Detection, and Estimation|last=Abraham|first=Douglas A.|date=2019-02-14|publisher=Springer|isbn=978-3-319-92983-5|language=en}}</ref> but may be in many different arrangements depending on what is being measured. As an example, in the article <ref>[https://www.bksv.com/en/knowledge/blog/sound/hydrophone-array Measuring noise at sea with hydrophone arrays systems]</ref> measuring propeller noise from fleet ships required complex hydrophone array systems to achieve actionable measurements.

[[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|Soviet]] submarines during the [[Cold War]] along a line from [[Greenland]], [[Iceland]] and the [[United Kingdom]] known as the [[GIUK gap]].<ref>Mackay, D.G. "[http://theses.gla.ac.uk/347/01/2008mackayMPhil.pdf Scotland the Brave? US Strategic Policy in Scotland 1953-1974]". ''Glasgow University, Masters Thesis (research).'' 2008. Accessed 12 October 2009.</ref> These are capable of clearly recording extremely low frequency [[infrasound]], including many [[List of unexplained sounds|unexplained ocean sounds]].

==See also==
*[[Communication with submarines]]
* [[Geophone]]
*[[Underwater acoustics]]
*[[Sonar]]
*[[Reflection seismology]]

==Notes==
{{Reflist}}

==References==
*Pike, John (1999). [[SOSUS]]. Retrieved 28 January 2005.
* Watlington, Frank (1979). ''How to build & use low-cost hydrophones.'' ({{ISBN|0830610790}})
* Unknown. [http://www.ob-ultrasound.net/hydrophone.html hydrophone]. Retrieved 28 January 2005.
* Unknown. (2005) [http://www.glossary.oilfield.slb.com/Display.cfm?Term=hydrophone Schlumberger Oilfield Glossary: Term 'hydrophone'] {{Webarchive|url=https://web.archive.org/web/20090418005434/http://www.glossary.oilfield.slb.com/Display.cfm?Term=hydrophone |date=18 April 2009 }}. Retrieved 28 January 2005.
* Onda Corporation (2015).  '[http://ondacorp.com/Handbook/ Hydrophone Handbook]'.
* Report AIR 1/645/17/122/304 - National Archives Kew.  Airship Hydrophone experiments.

== External links ==
{{Commons category|Hydrophones|Hydrophone}}
* [https://www.bksv.com/en/transducers/acoustic/microphones/hydrophones Hydrophones] —Brüel & Kjær Hydrophones and research articles
* [https://web.archive.org/web/20071022034253/http://dosits.org/science/measurng/1.htm DOSITS]—Hydrophone introduction at Discovery of Sound in the Sea
* [http://orcasound.net?07enohpordyhikiw10 orcasound.net]—Live hydrophone streams from killer whale habitat
* [http://pamguard.org Passive Acoustic Monitoring]—Using hydrophones to monitor underwater sounds
* [https://web.archive.org/web/20090122182929/http://sonar-fs.lboro.ac.uk/uag/products/products_hydrophone.html Build your own hydrophone]—free instructions
* [http://acoustics.co.uk/  Precision Acoustics]—useful resource on hydrophones
* [http://www.bl.uk/soundarchive The British Library Sound Archive]—contains many wildlife and atmospheric recordings made using hydrophones.
* [http://www.teledyne-reson.com/products/hydrophones/ High-Quality Hydrophones] {{Webarchive|url=https://web.archive.org/web/20150222214712/http://www.teledyne-reson.com/products/hydrophones/ |date=22 February 2015 }}— High quality manufacturer of Hydrophones. 
*[https://leaktronics.com LeakTronics.com] - Makers of Professional Swimming Pool Leak Detection Hydrophones 
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[[Category:Sonar]]