Guide · Dynamics
Sidechain Compression: The Pumping Effect, Explained
Sidechain compression is the technique behind the pumping you hear in most electronic music. It is also a genuinely useful mixing tool, and a genuinely divisive one. This guide explains what it actually does to the signal, so you can decide whether you want it.
What Sidechain Compression Is
A normal compressor has one input: the signal it is compressing. It watches that signal, and when it crosses the threshold, it reduces it.
A sidechain compressor has two inputs. The audio input is the signal being processed. The detector input is a different signal, and it is the one the compressor listens to. The compressor still reduces the audio input, but the decision to do so is made by the other signal.
In practice, this almost always means the bass triggers the compressor and the kick gets ducked. On every bass note, the kick is turned down a few decibels. The kick and the bass therefore stop competing for the same moment, and the bass is clearer every time it plays.
Why the Problem Exists in the First Place
A kick at 50 Hz and a bass at 50 Hz are the same note. No amount of level balancing fixes this, because the problem is not that one is too loud. It is that they occupy the same frequency at the same time, and the ear can only process one of them usefully.
This is frequency masking, and there are three ways to deal with it:
- Cut the bass so it sits above the kick, with a high-pass filter.
- Cut the kick so the bass owns the bottom, which loses the kick's weight.
- Time-share them, which is what sidechain compression does.
The third option is the interesting one. Instead of compromising the sound of either part permanently, you give each one the space it needs, briefly, every time it plays.
How to Set It Up
Most DAWs implement this as a compressor on the kick with its sidechain input set to the bass.
- Put a compressor on the kick. Not on the drum bus, on the kick channel or the kick bus, so that only the kick ducks.
- Set the sidechain input to the bass. A pre-fader send from the bass channel is the usual choice, because you want the bass to trigger the duck even when it is automated down in the mix.
- Set the ratio high. 8:1 to 20:1. A sidechain compressor is a trigger mechanism, not a levelling tool, and a gentle ratio will not produce a clear effect.
- Set the attack fast. 0.1 to 1 ms. If the attack is slow, the compressor misses the initial transient of the bass note and ducks late, which sounds wrong.
- Set the release to match the tempo. 80 to 200 ms at most dance tempos, so the kick comes back up before the next bass note.
- Set the threshold so that only the bass notes trigger it, and use the ducking depth to decide how much.
The depth control matters more than people expect. 3 to 6 dB is enough to create the effect while keeping the kick audible. Beyond 10 dB the kick vanishes on every bass note, which sounds dramatic and leaves a hole in the mix.
The Problem: It Only Works at One Tempo
A sidechain compressor is a timing device. It ducks the kick for a fixed length of time whenever the bass plays. That length has nothing to do with the tempo of the track.
If the track changes tempo, the ducking will no longer line up with the bass notes. The kick will be pulled down just after a bass note rather than during it, which sounds like a mistake rather than an effect. This is the single biggest limitation of the technique, and it is why a sidechained track that was written at one tempo and played at another often sounds worse.
If the track is going to be played at a fixed tempo, or is a finished piece rather than a live instrument, this does not matter. If it is not, sidechain compression is a risk.
When It Is Worth Keeping
The pumping effect is not a flaw to be corrected. In a lot of electronic music it is the point: it creates a physical sense of the kick driving the track, and it makes the low end feel enormous on small speakers, where a static balance would not.
The test is genre and context. In a house or techno record, a strong duck is a signature. In a pop record, a gentle one adds clarity without announcing itself. In a rock record, it will usually sound like a mistake.
The other argument for keeping it is that it solves a real problem. When a kick and a bass are fighting, the mix does not just get muddy, it gets confusing, because the listener cannot tell which part they are hearing. Ducking solves that without changing either part permanently.
When to Use Something Else
If the bass and the kick are fighting because they occupy the same frequency, sidechain compression treats the symptom. A high-pass filter on the bass, or a small EQ cut on the bass, fixes the cause and works at any tempo.
Try the filter first. Set a high-pass on the bass at 80 to 100 Hz and listen. If the conflict goes away, the problem was solved, and it will still be solved next year at a different tempo.
Reach for sidechain compression when the filter does not fix it, or when you specifically want the rhythmic effect.
Common Mistakes
Sidechaining the whole drum bus. This makes every drum duck, which is a bigger effect than most people intend and is usually an arrangement decision.
Using a slow attack. The compressor then reacts to the body of the bass note rather than its start, and the ducking arrives late and sounds like a flaw.
Sidechaining from a post-fader bass send. If the bass is automated down in the mix, the ducking stops with it, so the effect disappears at exactly the moments it is most needed. Use a pre-fader send.
Expecting it to sound good at a different tempo. It will not.
Questions
What is sidechain compression?
A compressor whose detector is fed by a different signal than the one it is compressing. The bass triggers the compressor, and the compressor ducks the kick. The result is that the kick makes room for the bass on every beat.
Does sidechain compression only belong in electronic music?
It is strongly associated with electronic music, but the technique is used in pop and rock too, usually more gently. A heavily pumping mix is a stylistic choice; a slightly ducked one is a mixing tool.
How much ducking is too much?
If you can hear the level moving on every hit, it is too much for most genres. A 3 to 6 dB reduction on the kick is usually enough to create the effect while keeping the kick audible. Beyond 10 dB the kick disappears.
Should I sidechain the whole drum bus or just the kick?
Just the kick, if the bass is fighting the kick specifically. Sidechaining the whole bus makes everything in the kit duck, which is a bigger and more obvious effect, and is normally an arrangement decision rather than a mixing fix.