Sidechain compression: making room without pulling faders
What a sidechain listens to versus what it compresses, the envelope shape behind the pump, the two uses people conflate, and the detector filter almost nobody touches.
The problem it solves
Two elements want the same space at the same time. Kick and bass fight over the low end; the pad swallows the vocal in the verses; the reverb covers the snare. The obvious solution is to turn one of them down, but then that element stays small for the whole song because of a conflict that only happens at specific instants.
Sidechain does the same thing, but only during those instants. The bass sits at full level throughout and steps aside only when the kick arrives. It's the difference between turning the TV down to talk and turning it down only while you're talking.
How it works underneath
Every compressor has two parts: one that listens and one that turns things down. In a normal compressor both get the same signal. The only difference with a sidechain is that the listening part is fed something else.
| Normal compressor | With sidechain | |
|---|---|---|
| Listens to | The bass | The kick |
| Compresses | The bass | The bass |
| Result | Evens out the bass itself | The bass steps aside when the kick hits |
What happens to the level across four beats has a specific shape, and that shape is the key to all of it:
The two uses, which aren't alike
Almost everything written about sidechain covers the first and assumes the second. They're different techniques with different settings.
| As an effect | To make room | |
|---|---|---|
| Goal | To be heard. It's part of the groove | To go unnoticed. Just resolve the clash |
| Reduction | 6 dB and up | 1 to 3 dB |
| Release | Long, tempo-synced | Short, just enough for the hit |
| Where | House, techno, electronic pop | Any mix, any genre |
The setting almost nobody touches
The detector filter. Many compressors let you filter what the detector hears without touching what gets processed, and that small detail completely changes the behaviour in two places.
- On the mix bus or master: without a high-pass on the detector, every kick makes the whole song breathe. With the filter at 100–150 Hz, the compressor stops obsessing over the low end and evens out what actually matters.
- On a sibilant vocal: filter the other way — letting only highs reach the detector — and you have a serviceable improvised de-esser.
- On a kick-triggered bass: high-passing the detector below 200 Hz makes only the kick hit trigger it, not the rest of the mix bleeding through the send.
When not to use it
- When the problem is frequency, not timing. If bass and kick cover each other because they occupy the same band constantly, EQ is what separates them. Sidechain doesn't fix continuous masking.
- When the clash happens three times in the whole song. That's automation.
- When the pattern is irregular. Sidechain shines against four-on-the-floor; over a broken rhythm it leaves the bass ducking in and out arbitrarily.
- When you've already added four of them. If every channel has its own sidechain, that wasn't the problem: the mix is overcrowded with elements fighting for the same space.
For the first one, see the kick and bass guide; for the second, volume automation.
Frequently asked questions
- What exactly is sidechain compression?
- A normal compressor listens to the same signal it's compressing. One with a sidechain listens to a different signal from the one it compresses: you put the kick in its ear and the bass through it, and the bass drops in level every time the kick hits. That's the whole idea. The rest is settings.
- How much gain reduction should I use?
- It depends on the goal. To make room without it being noticed, 1 to 3 dB is plenty; past 4 it starts reading as an effect. If the pumping is part of the sound — house, techno, much of electronic pop — 6 dB and up is normal and being noticed is the point. Decide which of the two you want before you touch the knob.
- What attack and release should I set?
- Attack as fast as the compressor allows: you want the gap already open when the hit lands, not after. Release is the setting that actually matters, because it's the one that draws the pump: it has to be back to full level just before the next hit. At 120 BPM that's around 400 ms for quarter notes; shorter sounds choppy, longer leaves the bass sitting dull.
- Sidechain or automation?
- For a regular, repeating pattern, sidechain: it does the work itself and adapts if the groove changes. For three specific moments in the song where a word gets lost, automation, because it's exact and leaves everything else alone. Using sidechain for a one-off problem is a sledgehammer.
- What is a filtered sidechain and why is it everywhere?
- It means filtering the signal the compressor listens to, not the one it compresses. High-pass the detector and the compressor stops reacting to low end, responding only to the rest. It's used constantly on the mix bus and the master: without that filter, every kick pulls the whole song down.
- Can I trigger it from something other than the kick?
- Yes, and that's where the technique earns its keep. Lead vocal ducking the guitars or a pad so the lyric lands without pulling faders. Snare opening a gap in the reverb. Bass controlling a synth's sub. The logic is always the same: whatever has to be heard is what does the triggering.