Side Chain Compressor: What It Does, Settings and Uses Beyond the Kick

A side chain compressor is a compressor that listens to one signal and turns down another. Feed the kick into its sidechain input and put it on the bass, and the bass dips every time the kick hits. Feed it a voice-over and put it on the music bed, and the music ducks whenever someone speaks. The compressor is ordinary; only the signal it listens to has changed.

This article explains how sidechain compression works and how to set it. For DAW-by-DAW routing steps, read how to sidechain, or go straight to sidechaining in FL Studio or sidechaining in Pro Tools.

What is sidechain compression?

Every compressor has two paths. The audio path carries the sound being compressed. The detector path, called the sidechain, measures level and decides how much gain reduction to apply. On a normal compressor both paths carry the same signal. Sidechain compression splits them: the detector hears a different source, called the key, and the audio path gets turned down whenever the key crosses the threshold.

That's why you'll see it labelled key input, external sidechain or side chain source, depending on the DAW. The idea is always the same: one sound controls another sound's level.

Sidechain compressor settings to start from

Threshold and ratio decide how deep the duck goes. Attack and release decide its shape, which is what you actually hear. These are starting points; adjust by ear with the full mix playing.

• Release sets the groove. For pumping, set it so the level recovers just before the next kick. Too long and the track never comes back up; too short and it snaps back with a click.
• Attack sets how much of the ducked sound's start gets through. For bass under kick, keep it fast so the kick's transient has the low end to itself.
• Watch the meter. Gain reduction is the real measure. Two compressors with different ratios can produce the same duck.

Uses for a sidechain compressor beyond kick and bass

Kick and bass is the famous one, but sidechain compression is a general tool for making two sounds share space.

• Voice-over and podcast ducking. Put the compressor on the music, key it from the voice, and the music drops while someone talks and rises in the pauses. Radio and video editors have done this for decades.
• Vocal against the band. Key a gentle compressor on the guitar or keys bus from the lead vocal. A dB or two of dip while the singer sings makes the vocal clearer without turning it up.
• Ducked reverbs and delays. Key the reverb or delay return from the dry vocal so the effect stays out of the way during words and blooms in the gaps. Lumen's Bloom and Drift's Duck control do this inside the plugin.
• Ghost-kick pumping. Feed the compressor a muted kick pattern so the pumping continues through a breakdown with no audible kick.
• Tightening two kicks or two basses. When layered low sounds smear, keying one from the other keeps one in charge.

Frequency-dependent sidechain: filtering the detector

Many compressors let you EQ the sidechain signal without changing the audio. That changes what the compressor reacts to, not how the track sounds.

• High-pass the sidechain on a mix bus. A high-pass filter around 80 to 150 Hz stops kick and bass from triggering compression on the whole mix, so the bus compressor stops pumping.
• Low-pass a full drum loop key. If you only have a drum loop rather than a separate kick, low-pass the sidechain so only the kick's energy triggers the duck.
• Band-pass for de-essing. Boost or isolate the 5 to 8 kHz area in the sidechain and the compressor reacts mainly to S sounds. That's how a simple de-esser works.

Ableton's Compressor and Glue Compressor, Logic's Compressor, Pro Tools' Dyn3 Compressor/Limiter and Reaper's ReaComp all include a sidechain filter or key filter for this.

Stock sidechain compressor plugins in each DAW

Our how to sidechain guide walks through each of these step by step, with screenshots of where the settings live.

Sidechain compressor vs volume shaper

A side chain compressor reacts to an actual signal. That's its strength and its weakness: it follows every ghost note and velocity change in the kick, and it stops when the kick stops. A volume shaper draws the same dip on a fixed rhythm locked to the tempo. It needs no routing and never changes from bar to bar.

Our free Pump plugin is a volume shaper. Rate, Depth and Curve set the classic duck, and the Custom shape lets you draw eight points for a duck no compressor would give you. Sidechain Snap below drops to silence at the start of each beat and climbs back almost fully by the second point, a tight, punchy duck.

Pump can also switch Trigger to Audio, restarting the shape on every kick or low hit in the audio it processes. That's useful on a drum bus or a full loop, where you want the duck to follow the real groove. Use a compressor when the duck must follow a separate track's uneven rhythm; use Pump for consistent, tempo-locked pumping. Many productions use both. For more on dynamics tools, see our compressor plugins guide and compressor vs limiter.