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What Is Sidechain Compression? And How To Use It

by Nick Messitte, iZotope Contributor August 20, 2021
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In mixing, you may have come across a form of compression called “sidechain compression” and had no idea what it meant. What is it and why is it useful? In this piece, we’re going to define sidechain compression for you in simple terms. We’ll show you some common uses, and point you to some examples of sidechain compression in the wild, so you can train your ears to recognize sidechain compression in action. 

In this piece you’ll learn

Want to follow along in your DAW? Try a free trial of Music Production Suite Pro membership to access Neutron Pro's sidechain compressor feature. 

What is sidechain compression? 

Sidechain compression is when the level of one instrument or sound triggers a compressor to act on another. We’ll get into a bit more detail later in this article about common methods of use, but one common use of sidechain compression is in electronic dance music (EDM). 

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

Before Sidechain Compression

After Sidechain Compression

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: 

Just Sidechain Pad

Compressors

Let’s start with the compressor. 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. 

Now, what is dynamic range? Most people use the term dynamic range to refer to the difference between quiet and loud sections in audio and musical programs. The phrase ‘Dynamic range’ actually indicates the difference in dB between the noise of an audio system and the level at which it distorts.

If the difference is too much, you can use a compressor to restrain the loudest bits, so that their amplitude doesn’t rise as high as it would un-compressed. This is one use for a compressor. 

Compressors are audio tools, and audio engineers are naturally curious people who love to experiment. So, compressors have been used, throughout the years, to alter music in colorful, impactful ways. 

Our goal is to introduce the concept of the sidechain, and build towards helping you understand sidechain compression in a holistic way.

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. 

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. 

The history of sidechain compression

A cinema sound designer named Douglas Shearer wanted to make some spoken dialogue quieter whenever the actor said a sibilant word. He wanted, basically, to de-ess the signal—only de-essers didn’t exist yet. 

So he invented one. He split the signal into two paths. One path went through the compressor, the other to an equalizer. With this equalizer, Shearer filtered the signal path until only its spikiest, most annoying ess sounds were prevalent. This signal was then routed into the compressor’s detector. 

The compressor “listened” to only the harshest sibilant sounds, and then “processed” the main dialogue signal. The result? The compressor only reduced the signal level when it detected harsh sibilant sounds.  

This is the sidechain in action. The sidechain bosses the compressor around in very specific ways—with very specific rules—and the compressor applies these rules to the main audio path. 

In the case of a de-esser, the rules are frequency-based: the boss is saying “only compress whenever you detect this super harsh frequency range,” and the compressor is saying, “aye aye, captain!”

Internal sidechain filters vs. external sidechain inputs

Are you with me so far? 

We’ve got the metaphor down—the sidechain has the ability to boss the compressor around—and we can move on to our next bit of business: internal sidechain filters versus external sidechain inputs.

These days it’s not uncommon for a compressor to have an internal sidechain filter. What does this mean? Within the compressor itself, you can tell the device or plug-in to ignore certain frequencies. You don’t need an external signal to boss it around. 

Say you were compressing an entire drum set. Well, that kick drum has a lot of energy. Perhaps it has so much energy that it triggers the compressor when you don’t want it to. 

An internal sidechain filter might come in handy here. It can help the compressor ignore the kick drum’s dominating low frequencies, filtering them out of the compressor’s detector circuit using an EQ.

A detector circuit, by the way, is just what it sounds like: it’s the signal path the compressor takes its orders from. 

So that’s an internal sidechain filter. Many digital compressors have internal sidechain filters, ranging from simple high-pass filters (removing extraneous low end, like kick drums), to emphasis filters (adding in tons of overbearing signal to force the compressor into action—like de-essers). 

Now, let’s cover the external sidechain input.

Neutron Pro Sidechain

Neutron Pro has a sidechain input, circled in the screenshot above. 

An external sidechain input allows you to use any audio signal whatsoever to boss the compressor around. 

Say you have a bass part and a kick drum. You want the kick drum to be audible, even when the bass is playing. Here an external sidechain input comes in handy. Simply follow these instructions:

  1. Slap a compressor on the bass

  2. Route the kick to the sidechain input (pictured above) 

  3. In the case of Neutron Pro, select “Ext Full” from the menu (also pictured above)

The compressor is now good to go: it will compress when it detects the kick, not the bass. The bass will duck down whenever the kick hits. 

That’s a typical use for a sidechain compressor. You can read about more creative uses here.

Common uses of sidechain compression 

Ducking basses to kick drums is quite popular in the world of music. It’s become a genre-defining aspect for much of EDM. You know it when you hear it: whenever the kick drum hits, much of the rest of the harmonic content “ducks” in response. This obvious pumping and breathing creates a fun and danceable effect. You can hear it in songs like “Are You Lonely” by Steve Aoki specifically in the drops.

For these next examples I can personally vouch for the sidechain compression used, as I mixed them. 

The first is a song by Peach Face called “No Windows.” In this tune, multiple bass parts and synth layers at the end of the song are side-chained to the kick.

A more subtle example of sidechain compression in action can be found in the tune “Another Friday Night News Dump” by Leland Sundries.

Here, we have an example of multiband sidechain compression: the electric bass is again tied to the kick drum—but only in the low end and low-midrange. When the kick hits, these frequency bands duck for a moment, making a subtle pocket for the kick drum.

I also use a fair amount of sidechain compression in my podcast mixes. 

In Salmon’s Run, Let Me Tell You About My Murder, Otzi: The Iceman Music Die, and True War Stories: Mission Report, I use multiband sidechain compression to help the voiceovers stand out over music beds. 

When someone talks, again, a subtle pocket is created, and they are able to be heard. Using this process in multiband means you don’t hear so much obvious ducking.

Experiment with sidechain compression

Sidechain compression is a valuable process to learn. It’s one of the most frequently used techniques in modern mixing, but beyond that, it really lets you do great work. Whether you’re trying to do something innovative or just trying to fit various different instruments into a mix, sidechain compression can be a big help. It’s also something that’s tricky to understand, especially when it comes to internal sidechain filters versus external sidechain inputs.

Use sidechain compression in your music with a free trial of Music Production Suite Pro

I hope this article demystified some of the processes for you. In the words of the great Biggie Smalls, if you don’t know, now you know! 

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