Hydrophone

A hydrophone (Greek "hydro" = "water" and "phone" = "sound") 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 in fluids. 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.

A hydrophone can "listen" to sound in air, but will be less sensitive due to its design as having a good acoustic impedance match to water, the denser fluid. Likewise, a microphone can be buried in the ground, or immersed in water if it is put in a waterproof container, but will give similarly poor performance due to the similarly bad acoustic impedance match.

History
The hydrophone was used late in World War I. Convoy escorts used them to detect U-boats, greatly lessening the effectiveness of the submarine. Ernest Rutherford, in England, led pioneer research in hydrophones using piezoelectric devices. His only patent was for a hydrophone device.

From late in World War One until the introduction of active sonar, hydrophones were the sole method for submarines to detect targets while submerged, and remain useful today.

Directional hydrophones
A small single cylindrical ceramic transducer can achieve near perfect omnidirectional reception. Directional hydrophones increase sensitivity from one direction using two basic techniques:

Focused Transducers
This device uses a single transducer element with a dish or conical-shaped sound reflector to focus the signals, in a similar manner to a reflecting telescope. This type of hydrophone can be produced from a low-cost omnidirectional type, but must be used while stationary, as the reflector impedes its movement through water.

Arrays
Multiple hydrophones can be arranged in an array so that it will add the signals from the desired direction while subtracting signals from other directions. The array may be steered using a beamformer. Most commonly, hydrophones are arranged in a "line array" but may be in two or three dimensional arrangements.

Links

 * DOSITS Hydrophone introduction at Discovery of Sound in the Sea
 * orcasound.net Live hydrophone streams from killer whale habitat
 * Passive Acoustic Monitoring Using hydrophones to monitor underwater sounds
 * Build your own hydrophone (free instructions)
 * Build your own hydrophone #2 Online tutorial at the freesound project.
 * Hydrophone recordings at the freesound project.
 * Precision Acoustics useful resource on hydrophones