Beyond The Basics: Sidechain Compression

For a video version of this story on sidechain compression, click here.

If you read SonicScoop, chances are you already know a thing or two about compressors. You’re probably already familiar with basic controls like your threshold, which allows you to set the point at which a compressor “kicks in”, and your ratio, which allows you to adjust the amount of compression you’ll get.

Early on into their training, most new engineers will also understand the concepts behind controls like attack, release and knee, which essentially adjust how swiftly your compressor reacts to signals that approach the threshold. (Even though it can take a while, sometimes years, to hear these variables well and to develop good instincts about tweaking them.)

But one of the last things that new engineers tend to explore when it comes to compression is sidechaining. For those who are unfamiliar with it, this is the process of using the output of one track to control the action of a compressor on a completely different track.

There was a time, decades ago, when many compressors lacked this function. But by the early 2000s, companies like dbx, Alesis and FMR had started to offer sidechain inputs on even their most affordable units. And today, a good DAW will allow you to add a sidechain input to almost any compressor, regardless of how simple its layout might be.

Sidechaining is a technique that can be used like a scalpel or a paintbrush, a hammer or a piece of fine-grain sandpaper. Today we’ll explore a few of its most common and potent applications.

Dialogue Ducking

In the beginning, one of the most common uses of a sidechain was a pragmatic one: Automatically reducing the level of music to make room for the human voice.

Today, most engineers mixing films and pre-recorded TV shows are likely to use volume automation to ride music levels, but sidechaining can still be handy, especially in live broadcast or event situations where music must make way for commentary.

Using a sidechain in this way is pretty simple. Just strap a compressor across your music track, and set it with a fairly low threshold, high ratio, a fast attack and a long release time. But, instead of the compressor reacting to fluctuations in the music, you’ll use the sidechain input to force the compressor to react to your dialogue tracks instead.

This way, whenever a voice enters the scene, the music is brought down, hard. If you set your release long enough, the background music will stay down between words and phrases, automatically making way for extended narration.

Vocal Mixing

This isn’t the only way to use a sidechain to make way for the human voice. In dense pop production, some mixers like to use  much more subtle version of this setup to clear space for a lead vocal.

In this scenario you’d likely want to go with a lower ratio and a faster release, so that the competing bits of music duck ever-so-slightly, almost imperceptibly, and then come back up to full volume rather quickly.

Instead of using your vocal track to trigger a compressor that’s strapped across the entire of the music mix,  you might target just an instrument or two that want to stay loud between vocal lines, but step just outside the spotlight whenever the singer makes an entrance. With a fast enough release, you can even set your compressor to let go in-between notes or phrases.

The goal with this approach is not to hear the music ducking, but to hear the vocal come through unobstructed – while leaving competing elements at an appropriately loud level in between. This can make for a subtly different effect than volume automation, as the volume dips and swells breathe with the rhythm of the voice. It can sometimes sound a bit more refined than automation, and it’s usually a little less tedious, too.

Punching Through a Pad 

One of the other most common ways to use a sidechain compressor in a music mix is to allow fleeting instrumental elements to penetrate through lush synths, string pads, or ever-present guitar tracks.

There are times where you may find that there’s an unacceptable tradeoff between crafting string pads and similar sounds to sound appropriately full, impressive and thick, and keeping them from obscuring other parts. In these cases, you can slap a compressor onto the instrument in question, and use a sidechain to trigger that compressor whenever the instrument it is masking is makes an appearance.

This way, you can give supporting sounds the expansive feel they deserve, and also let a fleeting instrumental element poke through, whether it’s an arpeggiated guitar, a busy bassline or the crack of a snare.

Clearing Room For The Kick

Perhaps the most common application — particularly in music with electronic elements — is to use a sidechain compressor to let  the kick drum to punch a little hole right through a bassline. To do this, simply insert your compressor on the bass, and use a bus to send your kick drum signal into the sidechain input.

Whether it’s a deep and viscous synth bassline or a super-busy bass guitar pattern, you can be quite subtle with this technique and still have it pay dividends. In some contexts, this approach can be far more powerful than EQing a kick drum to help it cut through the track. This way, there’s no need to compromise on the low end or to sculpt an overly “clicky” kick sound in order to let it poke through.

Letting it Pump

Of course there’s no law that says you have to be subtle. Whole genres of music have become obsessed with the radical use of this effect.

Set your ratio a little higher, your threshold a little lower, and your release a little longer, and you can with a kick drum sound reminiscent of the ones popularized by French house groups around the turn of the millennium. For an especially dramatic effect, don’t just use the sidechain to compress the bass track – You can use it to influence a compressor strapped across the entire mix.

Daft Punk’s “One More Time” and “Around The World” are often cited as some of the most iconic examples of a heavy handed approach to sidechained kick drum. (They used the sidechain input on a cheapo Alesis 3630.) By the mid-2000s, this kind of sound had become a significant flavor throughout American pop and EDM.

Used tastelessly, it can sound hopelessly dated. But for some types of music this fundamental technique borders on necessity. In EDM, it’s like a secret handshake. And in more experimental strains of rock, pop and Hip Hop, it can be used to tremendous effect without conjuring cliches.

UnGate It: Sidechain Compression in Reverse

Sidechains can be used to do more than just clamp down. Some engineers like to flip this arrangement around and to trigger a gate.

One popular application is to have your kick drum trigger a low frequency sine wave for extra weigh and some synthy, low-end resonance.

To do this, place a gate over a signal generator set to output low frequency sine wave (preferably in tune with the track) and then route your kick drum channel through a bus so that it opens a noise gate on your signal generator.

You can take this effect even further by playing a long-toned bassline on a synthesizer instead of using a fixed generator. The result is a musical bassline synced perfectly to your kick.

Tame That Frequency: A Band-Sensitive Compressor

You can also use a sidechain to tune a compressor so that it’s most effective around certain frequencies. This is essentially the concept behind a de-esser. To use a compressor in this way, feed a heavily EQ’d version of the track in question into your compressor’s sidechain input.

For instance, if you had a bass with one particularly out-of-control note, or a singer whose voice sounds overwhelming in a certain range, you could use an EQ to emphasize those frequencies, and then send that signal into the sidechain input. Conversely, if the low frequency content of a signal is causing a compressor to go haywire, a sidechain EQ can ensure that it only reacts to mid-frequency content.

Fortunately, many modern-style plugin compressors, like Pro Tools’ Dynamics III have a sidechain EQ feature like this built right in, and need no extra routing to achieve this effect. But if you want to use a vintage-style plugin or piece of hardware, getting up and running is not difficult at all.

If you think it through, you’ll come to realize that this kind of band-sensitive approach lies at the root of any multiband compressor. In fact, using sidechains and duplicate channels, you could feasibly create a multiband processor out of just about any plugin compressor you own.

Routing it All

In Pro Tools, use the “key input” dropdown (found at the top left of most dynamics plugins) in order to assign a bus to feed the compressor’s sidechain.

Setting up a sidechain can seem daunting if you’ve never done it, but it’s really pretty easy. Most hardware compressors will have a simple 1/4” TRS input that you can feed through a bus or aux send.

Inside of a DAW like Pro Tools, things are just as simple and perhaps even quicker. To make a kick drum the sidechain input of a compressor that’s strapped across a bass track, simply instantiate send on your kick drum kick track and route it to any unused bus – Let’s say bus 1. Just like you were going to send it out to a reverb.

Then, strap a compressor onto that bass track and set its sidechain input to bus 1. You can find this option next to the little key-shaped icon on the top left of a Pro Tools plugin, or next to the words “Side Chain” on the top right of a plugin in Logic. Then, feed an appropriate amount of signal and set your compressor to taste.

In the old days, sidechain inputs were largely seen as utility features, helpful in broadcast or for controlling sibilance in vocal tracks. But like always in the music world, once a tool is invented, someone’s going to find a way to use it creatively.

Applied with restraint, sidechains can be used to anchor the low end or to create multidimensional mixes, where subtle ducking helps usher sounds in and out of the foreground, creating a clear and powerful blend that constantly shift in focus. Used with abandon, a sidechain can introduce radical and unforgettable pumping effects, creating balances that are otherwise impossible to achieve.

There are many cases in which creative use of a sidechain might be woefully ill-advised. (For instance on a jazz recording or a rootsy Americana album.) But for engineers who work across genres, it’s just another essential technique to keep ready in the toolkit.

For a video version of this article on sidechain compression, click here.

Justin Colletti is a Brooklyn-based audio engineer, college professor, and journalist. He records and mixes all over NYC, masters at JLM, teaches at CUNY, is a regular contributor to SonicScoop, and edits the music blog Trust Me, I’m A Scientist.

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