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(hī`drəfōn'), device that receives underwater sound waves and converts them to electrical energy; the voltage generated can then be read on a meter or played through a loudspeaker. The hydrophone is the marine equivalent of the microphone, which receives and converts sound waves in air. It is used in sonar apparatus and in certain underwater weapons. The same device may also be used to generate sounds, converting electrical energy to motional mechanical energy; in this capacity it is called a projector.



a hydroacoustic sound pickup. Hydrophones are electroacoustic transducers and are used in hydroacoustics to listen to underwater signals and noises, for measuring, and as a component of directional hydroacoustic receiving antennas. The most common hydrophones are based on electrodynamic, piezoelectric, and magnetostrictive effects. The operating principles of an electrodynamic hydrophone are the same as those of electrodynamic air microphones, except for the design features needed to protect them from water.

Piezoelectric hydrophones utilize the direct piezoelectric effect of crystals such as Rochelle salt, quartz, ammonium dihydrogen phosphate, and lithium sulfate, in which a variable deformation of a crystal creates variable surface electrical charges and a corresponding variable electromotive force (emf) at the electrode plates. Piezoelectric ceramics such as barium titanate and lead zirconate-titanate are widely used. The sensing elements of piezoelectric hydrophones are made in the form of rectangular or cylindrical packets.

Magnetostrictive hydrophones are based on the inverse magnetostrictive effect of certain ferromagnetic metals (mainly nickel and its alloys), in which a deformation produces a variable magnetic induction in the magnetic circuit and, consequently, a variable emf in the winding. The sensing elements of this type of hydrophone (the cores) are generally made of thin lamina in order to avoid the losses caused by eddy currents.

Hydrophones intended for measuring purposes should be nondirectional and have a flat frequency response over the entire range of frequencies being studied. To this end it is expedient to use hollow spherical receivers made of a piezoelectric ceramic that are small compared with the wavelength and that perform spherical symmetrical vibrations.

One of the most important characteristics of a hydrophone is its sensitivity, which is the ratio of the electrical voltage to the sound pressure, in microvolts per bar (μV/bar); it lies in the range from fractions of a μV/bar for small ceramic spherical receivers (several mm in diameter) to hundreds of μV/bar for packets of piezoelectric crystals. Preamplifiers that are mounted in the same housing as the receiver and are immersed in the water with it are used to increase the sensitivity, as well as to avoid the shunting effect of cable.


Tiurin, A. M., A. P. Stashkevich, and E. S. Taranov. Osnovy gi-droakustiki. Leningrad, 1966.



(engineering acoustics)
A device which receives underwater sound waves and converts them to electric waves.