Sound & signal
Compressor
A compressor reduces the level of anything above a threshold, narrowing the gap between the loud and quiet parts. The result can then be turned up as a whole, which is how records get their density.
In practice
Compression is why a club record feels dense without any single moment being especially loud.
For a DJ the useful consequence is predictability: heavily compressed records are easy to gain-match and sit at a constant level through a blend, because there is little dynamic movement to track. A dynamic record — older, or deliberately spacious — needs more attention, since its quiet sections will disappear under a compressed one at the same nominal gain. Sidechain compression is the specific variant worth knowing: ducking everything on each kick is what produces the pumping motion in most house and trance.
Threshold
Where it starts working
Ratio
How hard it pulls
Attack
How fast it reacts
Where these come from: The three controls every compressor has, not catalog figures. For what we can measure, see sidechain compression, the DJ-visible version.
Why it matters
Compression is most of what makes a club record sound "produced". It is why the kick stays constant, why the vocal sits forward without ever jumping out, and why a track can feel enormous without any single moment being especially loud.
It is also the reason our energy reading behaves the way it does. Heavily compressed music has less variation to measure, so a whole-track average describes it well — and describes a dynamic record much less well.
Frequently asked
What does a compressor actually do?+
It turns down anything louder than a threshold, by an amount set by the ratio. The overall result is quieter but more even, so it can be raised again — which is how density is created.
What is sidechain compression?+
Compression triggered by a different signal, almost always the kick. Everything ducks on each kick and recovers between them, which produces the pumping motion that defines a lot of house and trance.
Do DJs need to use compression?+
Rarely, and not on the mix. The records are already compressed; adding more flattens what is left. It matters more as an explanation of why some tracks are easy to gain-match and others are not.
Related terms
Limiter
A limiter is a compressor with an absolute ceiling: it lets everything through until a set level, then refuses to exceed it. Mastering engineers use one on every record you own.
Ducking
Ducking pulls one signal down whenever another plays — almost always everything else dropping on each kick. It is the pumping motion at the heart of most house and trance.
Loudness
Loudness is a track’s average level in decibels below full scale, so every value is negative and closer to zero is louder. It decides how far you move the gain, not how hard the record hits.
Headroom
Headroom is the gap between where your signal is sitting and the point where it distorts. It is the room left for the loud parts, and for whatever you do to the channel next.
Explore: Loudness · How the analysis works · Limiter · All DJ terms