What is sidechain compression? And how to use it

Compression is one of the hardest things to hear when you first start mixing. But, there are so many ways to use compression that can make your mix have more impact. Sidechain compression is one of those ways. Sidechain compression is a mixing technique that deals with the volume relationship between two (or more!) tracks. Basically, as one track gets loud, the other gets quiet. But how does it work?

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What is sidechain compression? 

Sidechain compression is when the level of one instrument or sound triggers a compressor to control the level of another sound. The compressor's input is fed by the signal from the second track, also known as the sidechain, then compresses the first track in response, effectively turning down the volume of that track when the sidechain track is active. 

Hear the difference between an uncompressed track and a track that uses sidechain compression below.

To get a bit more granular, here’s an isolated audio example of the instrument that’s reacting to the input of the percussion to gain a better understanding of the processing: 

What is the sidechain?

In simple and analogic terms, you can think of the sidechain as the compressor’s boss: the sidechain is an aspect of the compressor that tells it how to react to the signal. 

A compressor is used to reduce a sound’s dynamic range – that is, to make the louder and quieter parts of the performance closer to each other in level, be it an instrument, a loop, or even a whole mix. 

The term sidechain is a shortening of the phrase “side signal chain.” “Side”, here, means “different from the main input source” (i.e., whatever you’ve slapped the compressor on). “Signal chain” refers to gear that processes an audio signal – such as an equalizer.

In fact, the first use of sidechain compression involved an equalizer. 

Using sidechain compression for practical purposes 

Sometimes also called a "ducker," sidechain compression can be used in practical ways such as ducking the volume of a bass track in response to the kick drum so that the kick drum can sound bigger by having less low-end competition. This is common in certain musical genres like EDM music. 

In radio or podcasting applications, you can use sidechain compression to lower the volume of a dialogue track each time the other person speaks or on the theme music track so that it ducks the volume of the music when the host starts talking. 

Typically, sidechains will only be found on dynamics processors such as compressors, gates, and dynamic EQs (though you may find exceptions). So, focus on threshold-based audio effects. 

Here are some other ways sidechain compression can be used in a mixing context: 

• Drum overheads with overwhelming snare: Toss a compressor on the overheads, then send the snare (close mic) to its sidechain. Every time the close-miked snare is played, the snare-heavy overheads will be attenuated.
• Bright guitars covering up the lead vocal: Insert a dynamic EQ or multiband compressor on the offending guitars. Set it to subtly turn down or compress the frequency range where the vocal is present and bright. You know what’s next; send the vocal to the sidechain! Whenever the vocal is heard, the guitars will get just a little bit “mellower.”
• Lots of bleed in the kick out: Put a gate on the “Kick Out” track. Assuming that the “Kick In” track doesn’t have excessive bleed, send it to the gate’s sidechain. Now, the kick track without bleed controls the gate on the bleed-heavy kick. The bleed in the “Kick Out” won’t open the gate!

Sidechaining for creative purposes 

Sidechaining can also be used in creative ways such as on the intro piano of "Everything I Wanted" by Billie Eilish. Notice how we perceive a beat even though there is no audible kick drum until about 40 seconds into the video.