NOISE IN BROAD-BAND HYDROPHONES

The sources of noise in underwater electro-acoustic hydrophones are considered with particular reference to piezoelectric piston-type elements. Expressions are derived for the various contributions, and their influence on hydrophone design for broad-band reception is discussed. It is shown that measurement of ambient noise levels down to Sea State 1/2 equivalent is possible up to 70 kHz with a correctly designed hydrophone and a low noise amplifier with sufficiently high input impedance. Measurements down to this level are more readily carried out with a spherical hydrophone than with a piston hydrophone in a baffle. The hydrophone parameters of greatest importance are the resonance frequency, coupling coefficient, and hydrophone sensitivity. It is important also to avoid mechanical resonances of the hydrophone mounting.