hydrophone
A hydrophone is a sound-to-electricity transducer for use in water or other liquids, analogous to a microphone for air. Note that a hydrophone can sometimes also serve as a projector (emitter), but not all hydrophones have this capability, and may be destroyed if used in such a manner.

The first device to be called a 'hydrophone' was developed when the technology matured, and used ultrasonic waves, which would provide for higher overall acoustic output, as well as increasing detection. The ultrasonic waves were produced by a mosaic of thin quartz crystals glued between two steel plates, having a resonant frequency of about 150 kHz. Contemporary hydrophones more often use barium titanate, a piezoelectric ceramic material, giving higher sensitivity than quartz. Hydrophones are an important part of the SONAR system used to detect submarines by both surface vessels and other submarines. A large number of hydrophones were used in the building of various fixed location detection networks such as SOSUS.