Compressor / Expander / Limiter / Gate

Compressor – Boosts low-level signals, and “squashes” high-level signals. The result is a consistent level of audio output regardless of the intensity (of lack thereof) of the input. Very useful in broadcast - for instance, the DJ can whisper or scream into the mic and you, the listener, don’t have to constantly grab the volume control; the level you hear remains constant. Also useful in live audio for the same reason, especially for vocals as well as other highly dynamic instruments like drums.

For the end user (that’s us), compressors are the “enemy of good audio.” Compressors are the reason that music on FM radio sounds flat and lifeless compared to the same music on a CD. Compressors are over-used in recording studios, IMO, and the result is the same – the end product does not sound nearly as realistic and dynamic as it could. The effect is especially prevalent on vocals and acoustic instruments - piano, drums and cymbals, etc.

Expander – A.k.a. dynamic range expander, pushes down residual noise floor in the absence of a signal. For instance, audible background noise between songs on cassette tapes can be virtually eliminated.

Limiter – Similar to a compressor, but evens out only high-level signals while imposing no effect on low-level signals. Typically used to prevent signal levels from exceeding a pre-set limit, to keep a sound system or the inputs of a device from clipping.

Gate – Effectively shuts off a signal when it falls below a pre-set threshold. Typically used on mics when the presence of a signal is absent. For instance, a speaker using a lapel mic: the mic is very sensitive and will pick up ambient room noise when the speaker pauses between phrases. The gate can be set for a low threshold to shut off when he is not speaking. Set the threshold too high and it will cut him off when he speaks softly. Another common use is for micing drums. In this case the threshold is set high, to pass the signal only when the drum is struck, but immediately close so as not to “bleed” - pick up the other drums as they are struck.

Peak Limiter – See “Limiter” above.

As you can see, only an expander would be useful in normal home audio. For amateur singers in karaoke, a compressor or limiter would be a big “plus” especially if they are going to be recorded. There is really no use for a gate outside professional applications.