Just as microphones collect sound in the air, underwater hydrophones detect acoustic signals in the ocean. Most hydrophones are based on a special property (piezoelecticity) of certain ceramics that produce a small electrical current when subjected to pressure changes. When submerged in water, a ceramic hydrophone produces small-voltage signals over a wide range of frequencies as it is exposed to underwater sounds propagating from any direction (read Ocean Acoustics for a discussion of how sound is produced and transmitted through the ocean). By amplifying and recording the electrical signals produced by a hydrophone, sound in the sea can be measured with great precision. Although a single hydrophone records sound arriving from any direction, several hydrophones can be simultaneously deployed in an array, and the resulting signals can then be manipulated to “listen” in any direction with even greater sensitivity than a single hydrophone element.
Whether within an array or as a single element, the hydrophone is the basic sensor of underwater acoustics. Currently, several technologies are available for acoustic exploration of the ocean.