Beyond The Basics: Sidechain Compression

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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 the compressor's sidechain.

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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