A Compander is a device that compresses the dynamic range of a signal before recording or transmitting it, and then expands it back to its original form during playback.

The name compander comes from the combination of the words compressor and expander. It solves the problem of trying to pass a signal with a large volume variation (wide dynamic range) through a medium that cannot support this variation (such as a cassette tape, which has a high level of background noise).

The process has two stages:

1. Compression (during recording): The compander compresses the signal. It increases the volume of the quieter parts and sometimes decreases the volume of the louder parts. This "flattens" the dynamics, ensuring that all audio is well above the background noise (noise floor) of the media.

2. Expansion (on playback): The compander performs the reverse process. It detects the compressed signal and expands it back to its original dynamic range, decreasing the volume of the parts that were increased.

By lowering the volume of the quiet parts back to their original level, the compander also lowers the volume of the tape's background noise, which becomes much less audible. The famous Dolby (A, B, C, S) noise reduction systems are the most classic examples of companders.