Hydrophones are underwater acoustic sensors, which most commonly use piezoelectric technology. They can be used singly or in arrays that can contain dozens or hundreds of individual units. They are used in marine geological and biological research, undersea mapping and navigation, and various commercial and military applications.

Critical specifications for hydrophones are frequency response, sensitivity and maximum operating depth. Frequency response is the range of frequencies for which the microphone maintains a constant sensitivity within defined boundaries. Variation from this "linear" or "flat" sensitivity is given in dB; bounds that define the linear frequency response will be in the form of ±3dB, -1dB/+4dB, etc. Typical values for sensitivity range between -220 and -150. More negative values represent lower sensitivity. This is measured in dB re V/microPA or microbar. The operating depth is how deep the hydrophones can be and still operate, as opposed to a survival depth, which will likely be much greater.

Hydrophones can have a number of features. An integral preamplifier boosts hydrophones’ output from a small to intermediate level so that the signal may be further processed without significant degradation. Acceleration noise compensation is designed to cancel or reduce noise generated from movement of hydrophones underwater. Hydrophones are also available in arrays. An array is a set of hydrophones wired together in series or parallel and typically towed underwater. In the case of arrays, sensitivity ratings are given for the individual hydrophones that are part of the array.

When referring to the weight of hydrophones, care should be taken to note that this specification is most commonly given as weight in air, but is sometimes seen as weight in water. When looking at temperatures for hydrophones, the distinction between operating and survival specifications is a critical one.