hydrophone
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 (emitter), but not all hydrophones have this capability, and may be destroyed if used in such a manner.

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. Contemporary hydrophones more often use barium titanate, a piezoelectric ceramic material, giving higher sensitivity than quartz. 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.

A hydrophone is a microphone designed to be used underwater for recording or listening to underwater sound. Most of them contain a piezoelectric transducer that converts the a pressure change into electricity. They convert a sound signal into an electrical signal. Some may also be used as projector (emitter) . This equipment has acoustic impedance match to water and may not be sensitive to sound in air. if it is put in a waterproof container also the same case appears. The hydrophone was used late in World War I.They were widely used for the detection of U-boats. hydrophones using piezoelectric devices was pioneered by ernest rutherford.

Directional hydrophones
They have better sensitivity from one direction due to the usage of the following techniques:
-Focused Transducers:
a dish or conical-shaped sound reflector is placed to focus the sound signals.

-Arrays:
Multiple hydrophones can be arranged in an array so that the signals recieved by each will be added up and the signals from other directions get subtracted. The array may be steered using a beamformer.

Visit these liunks fo rmore details:
http://en.wikipediawiki/Hydrophone
http://ondacorpimages/brochures/Onda_HNR_DataSheet.pdf
http://marinetechrov_competition/Hydrophone.pdf