Editing Hydrophone (section)

==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 of the equipment.{{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, and so 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|in|frac=32|adj=mid|-thick|spell=in}} 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.