What Is Sidechain Compression? And How To Use It
by Nick Messitte, iZotope Contributor August 20, 2021
Learn about sidechain compression in audio plug-ins and how to use it effectively.
Sidechain compression is when the level of one instrument or sound triggers a compressor to control the level of another sound. One common use of sidechain compression is in electronic dance music (EDM). A compressor is placed on the bass so when there’s a kick drum hits, the bass level drops to create room for the kick.
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
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!”
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 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:
- Slap a compressor on the bass
- Route the kick to the sidechain input (pictured above)
- 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.
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. To get started, here are some frequently-encountered situations that can benefit from sidechain processing.
- Bass drowning out the kick: Using the configuration detailed in the previous section, the bass will be compressed each time the kick hits. As a result, the kick will be more audible than the bass during those hits. This can be a great way to get the kick to punch through a bit more, especially if your kick and bass share the same frequency range.
- 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. If there is significant kick bleed in the close snare mic signal, use a high-pass filter on the sidechain to minimize it!
- 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!
- Fiending for some ‘80s snare reverb: Set up a reverb plug-in on a spare channel. Insert a gate directly after the reverb plug-in. Send the snare track to the reverb channel and to the sidechain of the gate. Ho ho ho, dear readers. What happens as a result is not for seekers of the subtle. Each time the snare plays, the gate will open and let the reverb be heard, and the gate will close after each crack of the snare. So, the existing of snare signal triggers the reverb, and the lack of snare signal stops the reverb (abruptly or gently, depending on the gate settings). Wild and nostalgic!
- Ducking basses to kick drums: This 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.