What is sidechain compression?

Sidechain compression in plain English

A normal compressor listens to the sound going through it and turns that sound down when it gets loud. Sidechain compression splits those two jobs. The compressor still sits on one track, say the bass, but it listens to a different track, say the kick. Whenever the kick goes over the threshold, the bass gets turned down, and when the kick fades the bass comes back up.

The sound you hear changing is called the target. The sound doing the controlling is the trigger, and the input it arrives through is the sidechain. The trigger itself isn't changed at all; it only tells the compressor when to act.

In the lab above, the purple lane is the trigger and the pink shape is the target. Each time a purple kick crosses the threshold, the amber lane dips and the pink shape shrinks with it.

What is a sidechain?

Every compressor has two paths inside it. The audio path is the sound that gets turned up or down. The detector path, called the sidechain or key input, is what the compressor measures to decide how far to turn it down. On an ordinary compressor both paths carry the same sound, so a loud vocal turns itself down.

An external sidechain feeds the detector from somewhere else. The audio path still carries the bass, but the detector now hears only the kick, so the bass's own level has no say in it. A quiet bass note under a loud kick gets ducked just as hard as a loud one.

Plenty of compressors also let you filter the sidechain without feeding it a different sound. The detector hears an EQ'd copy of the signal, for example with the low end cut so a bass-heavy mix won't set the compressor off on every note. People call that sidechaining too, which is why the word turns up in places that have nothing to do with kick drums.

Ducking and pumping

Both words describe the same process at different strengths. Ducking is the practical version. The bass drops 3 or 4 dB for the first 100 milliseconds of each kick, which is long enough for the kick's punch to come through and short enough that nobody notices the bass moving. Used like this, you'd only notice it by switching it off.

Pumping is the effect you're meant to hear. The ratio goes up, the duck gets deeper, often 10 to 20 dB, and the release gets long enough that the target swells back up across the whole beat. Pads or the whole mix seem to breathe in time with the kick. It's a big part of the sound of French house and a lot of EDM.

Outside music production, ducking is all over broadcasting. A radio presenter talks and the music drops under their voice; they stop and it comes back up. That's a sidechain compressor with the voice as the trigger and the music as the target, set with a slower attack and a release of several hundred milliseconds so the music doesn't jump around between words.

Why the kick and bass fight

A kick drum puts most of its weight somewhere between 40 and 100 Hz. So does a bass line, especially in electronic music where the bass is often a sine or a filtered saw sitting on its fundamental. When both play at the same moment their waveforms add together.

Say the kick peaks at −6 dBFS and the bass sits at −8 dBFS. Where their peaks line up, the sum reaches about −1 dBFS and eats your headroom; where they're out of step, they partly cancel. Which of the two you get depends on the bass note, so it changes from beat to beat. The low end goes woolly and the kick loses its shape. Turning the bass down for the first fraction of each kick gives the kick a clean slot and the bass the rest of the beat.

Step 1 in the lab plays the two with no ducking. Listen to the first moment of each beat, then go to step 2 and listen again. The kick gets clearer without getting any louder, because the bass is moving out of its way.

The kick and bass lesson covers the rest of getting these two to sit together, with EQ and with tuning.

Reading the lab

The big screen shows the whole four-bar loop. The vertical lines are beats at 120 BPM, with a brighter line at the start of each bar. The purple lane at the top is the trigger. Under it, the pale shape is the target before the compressor and the pink shape is what comes out. The amber lane at the bottom shows how far the target is being turned down at every moment, which is the compressor's gain reduction. Hover over it, or press and drag on a phone, to read the number at any point.

The square screen shows how far the target ducks for each level of the trigger. It's flat at 0 dB until the trigger reaches the threshold, then slopes down, and the higher the ratio the steeper the slope. Next to it is one beat up close, with the attack time shaded and a dashed line where the target is back to within 1 dB of full level, if it gets there before the next beat.

Above the screens you choose the target (bass, chords, strings or the full mix) and the trigger. The Kick option is a plain kick on every beat, made in your browser; Drum loop uses the whole loop from Wonky Percussion. A/B switches the sidechain off at matched loudness, so the only thing that changes is the ducking. You can also upload your own sound or record one, and the lab will duck it under the trigger.

The controls

These are the controls you'd find on any compressor. The difference is that threshold and ratio now respond to the trigger's level, while attack and release shape the way the target moves.

Threshold

The threshold is the trigger level where ducking starts. The kick in the lab peaks at about −1 dB, so with the threshold at −24 dB it goes 23 dB over. Lower the threshold and each kick is further over, so the duck gets deeper, and it stays over for longer as it fades, so the duck lasts longer too.

With a drum loop as the trigger, the threshold also decides which hits count. At −12 dB, only the loudest hits in the Wonky Percussion loop get over it. Drop it to −30 dB and the quieter snares and hats join in, until the target hardly gets a moment at full level.

Ratio

The ratio sets how hard the target gets turned down for each decibel the trigger goes over. At 4:1, a kick 20 dB over the threshold turns the bass down by 15 dB, because 20 dB over comes out as 5 dB over and the other 15 dB is taken off. At 12:1 the same kick takes off about 18 dB. Above 10:1 the duck is close to a full drop, which suits a pump and is too much for a duck you aren't meant to hear.

Attack

Attack is how fast the target ducks once the trigger crosses the threshold. For kick and bass you'll usually want it fast, between 0.1 and 1 ms, so the bass is out of the way before the kick's transient, its first click, has finished.

A slower attack lets the start of each beat through with the target still at full level. On a bass that means the kick's click has to compete again, which is usually the opposite of what you wanted. On pads it can sound smoother, because the duck starts gently. Step 4 in the lab sets a 30 ms attack, and in the close-up you can see the pink shape staying tall for the first part of the beat.

Release

Release is how quickly the target comes back up after the kick, and it decides whether you get a duck or a pump. The time between kicks comes from the tempo: one beat lasts 60,000 divided by the BPM, in milliseconds.

If the target is back to full level well before the next kick, you hear a short duck and then the note again. If it's still on its way up when the next kick arrives, it never reaches full level, and that constant rise and fall is the pump. On most compressors the release time is how long the gain takes to recover most of the way, and it can't finish recovering until the trigger has dropped back under the threshold. In the lab, at 12:1 with the threshold at −24 dB, a 150 ms release still leaves the bass about 1 dB down when the next kick lands, and a 250 ms release leaves it more than 3 dB down. The figure below runs the lab's compressor over one bar so you can see where that point falls.

Very short releases, under about 40 ms, recover so fast that on low notes the compressor can follow the kick's own waveform up and down, which adds a faint buzz to the bass. Start around 100 ms for a duck and 200 to 400 ms for a pump at 120 BPM, then adjust by ear.

Trigger level and the silent trigger

The Trigger control in the lab is the kick's own fader: how loud the kick is in what you hear. The compressor listens to the kick before that fader, so turning it down, even all the way to Off, doesn't change the ducking. What's left when it's off is the pump with no kick playing.

Producers do this on purpose. You make a kick track that plays on the beats you want, feed it to the sidechain and keep it out of the mix. The target pumps in time even in a breakdown where the real kick has stopped, or when the real kick plays a rhythm you don't want the bass to follow. You'll often see it called a ghost kick.

Listen to the trigger

Many sidechain compressors have a button that plays the sidechain signal on its own, so you can hear exactly what the detector hears. It's most useful when the trigger isn't a clean kick. A drum bus as the trigger includes the snare and hats, and a filtered sidechain might be missing more than you thought.

Switch the lab's trigger to Drum loop and you'll see why most producers trigger from the kick alone. Every hit that crosses the threshold ducks the target, so the bass moves with the snare and the busier hits as well as with the beat.

How to set up sidechain compression in any DAW

The menu names change from one DAW to the next, but the steps are the same everywhere.

1. Put a compressor on the targetInsert it on the track you want to duck, such as the bass or a pad. The kick track doesn't get a plug-in.
2. Route the trigger into the compressor's sidechainFind the compressor's sidechain input and choose the kick track as its source. Most DAWs list your tracks in a menu on the compressor itself.
3. Start with a firm ratio and a fast attackA 4:1 ratio and an attack around 1 ms make the duck easy to see while you set the rest.
4. Lower the threshold until the meter moves on every kickWatch the gain reduction meter. Aim for 3 to 6 dB for a hidden duck, or 10 dB and more for a pump.
5. Set the release against the tempoWatch the meter between kicks. Shorten the release if it never gets back to zero and you didn't want a pump; lengthen it if the duck sounds like a click.
6. Check it at matched loudnessDucking lowers the target's average level, so nudge the target's fader up until the bypassed and processed versions sound equally loud before you decide.

Sidechain compression in Ableton Live

Put Live's Compressor on the bass track and open its sidechain section with the small triangle button at the top of the device. Switch Sidechain on, then pick the kick track in the Audio From menu. The Compressor's own meter shows the gain reduction on every kick, and the headphones button in the sidechain section lets you hear the sidechain signal on its own, which is the same job as Listen to trigger in the lab.

Sidechain compression in Logic Pro

Insert Logic's Compressor on the bass channel. At the top of the plug-in window there's a Side Chain menu; choose the kick's track there. Logic's Compressor then reacts to the kick, and its gain reduction meter shows each duck.

Sidechain compression in FL Studio

In FL Studio the routing happens in the Mixer. Select the kick's mixer track, right-click the routing arrow at the bottom of the bass's mixer track and choose Sidechain to this track. Then put Fruity Limiter on the bass track, switch it to its COMP mode and set its sidechain input to the kick's mixer track number.

Volume shaping: ducking without a compressor

The other way to get a pump is to draw it. A volume shaping plug-in, such as Xfer's LFOTool or Cableguys' VolumeShaper, turns the target down along a curve you draw, repeating in time with the project tempo. You can get the same result with volume automation, or with an LFO assigned to a track's volume in DAWs that allow it.

Volume shaping gives you exactly the same curve on every beat and needs no trigger, so it's handy for a pump through a breakdown or a shape no compressor would make. A compressor reacts to the real kick instead: it follows fills, ghost notes and changes in the pattern, and its duck changes with the kick's level. For kick and bass, start with a compressor so the bass follows what the kick actually does, and reach for volume shaping when you want a fixed, designed pump. The two are compared side by side in sidechain vs volume shaping.

Starting settings for sidechain compression

These assume a kick on every beat at around 120 BPM. Scale the release with the beat length if your tempo is a lot faster or slower.

A ducked reverb is worth trying on any vocal. The reverb gets out of the way while the singer is singing and blooms in the gaps, so you can use more of it without the words getting washed out. The same idea applied to an echo is covered in ducked delay.

Common sidechain mistakes

Ducking so far the bass disappears

A deep duck with a long release can leave the bass turned down for most of every beat. Check the meter just before each kick: if it hasn't come back near zero and you weren't going for a pump, raise the threshold or shorten the release. In the lab, the At next hit figure tells you how far down the target still is when the next hit lands.

Using the whole drum bus as the trigger

Feeding the drum bus to the sidechain is quick to set up, but every snare and hat that crosses the threshold ducks the target as well. Use the kick on its own track, or a ghost kick, so the target moves on the beats you mean.

A release that ignores the tempo

A release copied from a tutorial at 128 BPM can sound wrong at 90 BPM, because there's about 200 ms more between kicks. Work out the beat length first and set the release against it.

Turning the kick down and losing the duck

Some routings take the sidechain signal after the trigger's fader, so lowering or muting the kick lowers the duck with it. If the pump goes away when you turn the kick down, change where the sidechain is taken from, or use a separate ghost kick that never changes level.

Judging it by loudness

Ducking lowers the target's average level, so the sidechained version can sound weaker in a quick comparison simply because it's quieter. The A/B in the lab evens that out. In your DAW, match the levels by ear before deciding.

Train your ears

Gentle ducking is meant to be hard to hear, which is exactly why it's worth practising. The quiz under the lab plays the bass with the kick and hides the controls. The first four rounds ask whether the sidechain is on, and the last four ask whether the release is fast or slow. The lab matches the levels in every round, so you have to listen to how the bass moves, because its loudness gives nothing away.

Listen to the bass straight after each kick. A fast release sounds like a short gap; a slow one sounds like the bass swelling back up across the beat. Your best score is saved in your browser, so once you're getting most rounds right, try it again on the chords or the full mix.

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