A hydrophone is a sound-to-electricity transducer for use in water or other liquids, analogous to a microphone for air. Note that a hydrophone can sometimes also serve as a projector (emittor), but not all hydrophones have this capability, and may be destroyed if used in such a manner.
History
The first working sonar was built by Reginald Fessenden in the United States in 1914. It used an electromagnetic moving-coil oscillator that would emit a low frequency noise, then switch to listening mode to receive the echoes. Because of its crude technology, it could not precisely determine direction.
The first device to be called a "hydrophone" was developed when the technology matured, and used ultrasonic waves, which would provide for higher overall acoustic output, as well as increasing detection. The ultrasonic waves were produced by a mosaic of thin quartz crystals glued between two steel plates, having a resonant frequency of about 150 KHz.
Hydrophones are an important part of the SONAR system used to detect submarines by both surface vessels and other submarines. A large number of hydrophones were used in the building of various fixed location detection networks such as SOSUS.
Directional hydrophones
A single cylindrical ceramic transducer can achieve near perfect omnidirectional reception. Directional hydrophones increase sensitivity from one direction using two basic techniques:
Damping and reflecting
This method uses a single transducer element with a dish or conical-shaped sound reflector to further focus the signals. 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. Most commonly, arrays are arranged in a "line array."
Geophysics
Hydrophones are used by geologists and geophysicists in detecting seismic energy. They are combined to form streamers that are towed by seismic vessels or deployed in a borehole.
References