Hydrophone

Most hydrophones contains a piezoelectric 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.

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 American Submarine Signaling Company developed a hydrophone to detect underwater bells rung from lighthouses and lightships.

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.

[8] In 1918, airships of the Royal Naval Air Service engaged in anti-submarine warfare experimented by trailing dipped hydrophones.

[10] 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 submarines during the Cold War along a line from Greenland, Iceland and the United Kingdom known as the GIUK gap.