Overview

All the applications of underwater projectors described at the beginning of Chapter 3 also require the use of hydrophones. In most active sonar systems the same transducers serve as both projectors and hydrophones, but there are good reasons in some cases to use separate hydrophones for reception, (e.g., hydrophones in towed line arrays can be well removed from the ship's self-noise). In addition passive search and surveillance sonar, as well as passive ranging sonar, use only hydrophones. Passive sonobuoys and various noise monitoring functions also require only hydrophones.

Hydrophones detect the pressure variations of acoustic signals and noise in the water and produce an output voltage proportional to the pressure. In addition they generate a noise voltage due to thermal agitation in any internal resistances. Thus the performance criteria for hydrophones are quite different from those for projectors. While projectors are usually operated in the vicinity of resonance, with power output as the major concern, hydrophones are usually operated over a wide band below resonance, and the open circuit output voltage and signal-to-noise ratio are of most concern. The smallest signal detectable by a hydrophone is equal to or slightly less than the ambient sea noise unless the internal hydrophone noise plus the preamplifier input noise exceeds the sea noise. The noise voltage generated by a hydrophone and its preamplifier noise may be compared with the sea noise by relating it to an equivalent noise pressure in the water using the hydrophone sensitivity.

Hydrophones are usually smaller and simpler than projectors,...