A hydrophone (Greek "hydro" = "water" and "phone" = "sound")'s 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's a pressure wave in fluids. Some transducers can also serve as a projector (emitter), but not all have this capa… (More on Hydrophone)