Sidechain compression sounds like a technical secret, but the idea is simple: one sound tells a compressor to turn another sound down. You hear it in almost every dance track, where the chords seem to breathe with the kick, and in a lot of pop and rap, where you hear nothing but a cleaner low end. Once you understand what is happening, you can use it with taste instead of copying a preset.
What a normal compressor does, and what changes
A normal compressor listens to the sound it sits on. When that sound gets louder than a set level, the compressor turns it down. That is how it evens out a vocal or a bass.
A sidechain compressor listens to a different sound. It sits on one track but takes its trigger from another. The classic setup is a compressor on the bass whose sidechain input is the kick. Every time the kick hits, the compressor turns the bass down for a moment, and when the kick is gone the bass comes back.
Nothing about the bass itself is changed. Only its volume moves, in time with the kick.
Why kick and bass
The kick and the bass live in the same low frequencies. When they hit together they fight for the same space. The result is a muddy low end, a weaker kick and less room in the mix, because low frequencies use up a lot of your headroom.
If the bass ducks for a split second whenever the kick hits, the kick punches through clearly, and the bass returns right after. You do not hear a gap, because the ear is busy with the kick. You just hear a tighter, louder, cleaner track.
The pumping effect in dance music
In dance music the effect is exaggerated on purpose. The ducking is deep and the release is timed to the tempo, so that a pad, a synth or the whole bass section swells back between the kicks. This is the "breathing" or "pumping" that gives house and electro their movement.
Starting points for a pumping sound, to adjust by ear:
- Threshold: low enough that the compressor reduces the signal strongly on every kick.
- Ratio: high.
- Attack: as fast as it goes, so the duck starts exactly on the kick.
- Release: set so the volume is back to normal just before the next kick. If the kick falls on every beat, that is a bit under the length of one beat.
Change the release until the pumping follows the groove. If it feels late, shorten it. If it feels choppy, lengthen it.
Subtle use in pop and rap
In most pop and rap nobody should notice the pumping. The aim is clarity. A bass, a guitar or a pad ducks by a small amount under the kick, and the release is short. The track simply feels cleaner.
In rap this often means the 808 and the kick. When they overlap, the low end turns into a blur. A light sidechain from the kick to the 808, with a quick release, helps. Some producers shorten the 808 tail or use a high-pass on one of them instead, and both are fine. Pick what sounds cleaner.
Another small trick is to send the vocal to the sidechain input of a compressor on the beat, and to duck the beat by one or two decibels when the voice is there. The voice stays in front without being pushed louder.
Three ways to do it
- A compressor with a sidechain input. Most DAWs have one. Put it on the bass, then choose the kick as the sidechain source in the plugin.
- A volume shaper. These plugins draw a volume curve in time with the tempo, so you need no kick as a trigger. The effect is the same every bar.
- A "ghost kick". Create a silent kick track that only triggers the compressor. It is useful when the audible kick pattern is irregular, when the kick is muted in a section, or when you want the pump on a different rhythm.
Names and menus differ between DAWs, so check how yours routes a sidechain.
Common mistakes
- Too deep. The bass disappears every time the kick plays. Reduce the amount until you only just feel it.
- Release too long. The bass does not recover before the next kick, and the track sounds weak and sleepy.
- Release too short. You can hear clicks or a wobble in the low end.
- Ducking the whole mix when only the bass needed it.
- A silent trigger. If you mute the kick and the ducking stops, route the sidechain send before the fader, or use a ghost kick.
- Judging it solo. Listen with everything playing, then on headphones and on a phone speaker.
A simple test: turn the effect off and on while the full track plays. If you miss it when it is off and do not notice it when it is on, you have set it well.