A device that automatically reduces the range of amplitude variations of an audiofrequency signal in a transmission system. It operates by decreasing the amplification of the signal when it has a value greater than a predetermined amplitude and increasing the amplification when the signal amplitude is less than a second predetermined value. A volume expander is a device that produces the opposite effect, i.e. it automatically extends the range of amplitude variations of the transmitted audiofrequency signal.

A suitably designed expander used at one point of a transmission system can be made to compensate for the effect of a compressor in another part of the system and thus restore the original audiofrequency signal. A compressor and expander used together in this manner are termed a compandor.

In the recording of sound a compressor may be used to reduce the volume range of the signals recorded on a gramophone record or on a film track where the dynamic range that can be recorded is less than the dynamic range of the sound to be recorded. The sound-reproducing apparatus will then include an expander to compensate.

A telecommunication system, such as a radio-telephone system, often employs a compandor to improve the signal-to-noise ratio of the system; the compressor is used at the transmitter and the expander at the receiver. The relative increase in the smaller transmitted signals reduces the effect of noise on these signals.