Intro to Sidechain Compression

Of all the advanced techniques in a mixer’s toolkit, sidechain compression is arguably one of the most useful – and yet often the one that gets overlooked or misunderstood. Like most processing effects, you can live without using sidechain compression, and even get great mixes, but once you understand it, it’s hard to go back to working without it.

We’ll go over sidechain compression basics here, so you can get started implementing it in your mixes (if you haven’t already).

Compression and Sidechaining

As a simple reminder, compression reduces the dynamic range of a signal, usually by setting a loudness threshold and reducing any part of the signal that goes over that level. In a normal compression situation, the compressor responds to the same signal it’s processing.

Sidechaining is a technique which allows you to process a signal in response to a different signal. So, sidechain compression means setting up a compressor to reduce the level of the signal you want to process in response to the level of another signal.

For example, if you apply a normal compressor to a bass track, when the bass goes past the threshold, the bass is reduced. If you were to instead use a sidechain compressor side chained to a kick drum, say, the bass will be reduced when the kick drum goes past the threshold.

This is also known as ducking, and a common use is to automatically duck music beds when voice over or DJs start talking in radio.

In music mixing, sidechain compression is a great way to get clashing elements out of each other’s way. Probably the most common use is to duck the bass just a little every time the kick hits. Although the same concept can be applied to anything that clashes. For example, there are often several instruments other than bass - like pianos and guitars - that mask the kick a bit. Similarly, guitars and mid-range piano parts can mask vocal tracks. All these are good candidates for sidechain compression.

Using Sidechain Compression

Every hardware or plugin compressor is different, and every DAW handles sidechaining differently. Some compressor plugins and hardware compressors simply don’t have the capability to sidechain. If they do, however, the basic procedure is this:

• Set up a sidechain-capable compressor on the track you want to duck (eg: the bass in the example above)
• Route an extra output from the track you want to affect the compressor to the sidechain input of the compressor. This is usually done by creating an external send from the master track to the process track (eg: the kick from above).
• Set the compressor to sidechain mode so it knows to react to the master track level and not the processed track.

That’s it! Adjust threshold, ratio, attack, and decay until you get what you need. You’ll want to use an external send rather than routing the main output of your master track to the sidechain so you can still hear the master track. Unless you don’t want to hear the master track…

Creative Sidechain Uses

Sidechaining has been around forever, but in recent years it’s taken on a different life for EDM producers. Often, EDM makes use of sidechaining for creative purposes rather than just technical. For example, you can side chain the entire mix to the kick drum to make the mix pump with the kick.

Or you could set up a synth drone that on its own would just be a long flat note, and sidechain a rhythmic input to it, ducking it almost all the way out, to create a rhythmic synth track. This is often done with a dummy rhythm track – one that isn’t intended to be heard in the mix. In this case, instead of using an external send, you would send this rhythm master track’s main output to the sidechain input on the compressor.

Bandlimited Sidechaining

Finally, you may notice as you experiment with sidechain compression that the effect is a little too much. This often happens when sidechaining bass to kick drum. In this case, you might want to duck only a certain frequency range.

You can accomplish this in one of two ways: with a multiband compressor or a dynamic EQ. In a multiband compressor, you would follow the same procedure, but route the sidechain input to only one band, or set all other bands to do nothing.

In the other way, you’ll need an EQ capable of dynamic EQ, which is an EQ which will duck certain frequencies when they get past a threshold – it’s very similar to a compressor. You’d set up the sidechain to route to one of the EQ’s bands, set up the cut, and voila. In the bass/kick example, a good starting point is to setup a low shelf below 100Hz on the bass that only engages when the kick drum hits. This way only the frequencies that mask the kick are affected, which keeps the bass in general sounding more even.

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