Hydrophone

Please note that the content of this book primarily consists of articles
available from Wikipedia or other free sources online. A hydrophone 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 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. 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.