hydrophone

A hydrophone () is a microphone designed for underwater use, for recording or listening to underwater sound. Most hydrophones contain 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. Sound travels 4.3 times faster in water than in air, and a sound wave in water exerts a pressure 60 times more than what is exerted by a wave of the same amplitude in 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.