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Along with noise, reverberation - the backscattered part of the total reflection - forms the background to the detection of targets by an active sonar. In many environments, reverberation can be the dominant component of this background. It is therefore very important for the design of active sonars to have a good measure of its magnitude and how it varies with roughness and grazing angle.

A passive circuit produces a noise proportional to the resistive component of its equivalent impedance, even when this corresponds to a physical reality far removed from a simple resistance. For an aerial, a component of its impedance corresponds to the coupling with the surrounding space and to exchanges of energy with this space. This component is known as the radiation resistance of the aerial.