Sidechain compression is a very common, but often considered “advanced,” mixing technique. If you’ve made it this far to be curious about it, you’re probably pretty familiar with how compressors generally work. If not, there’s always time for a quick refresher!
Compressors are dynamics processors, meaning they control the volume envelope of a sound. In essence, a compressor takes a wide dynamic range and makes it smaller. You can think of it as reducing the loudness of peaks and increasing the quieter parts of a signal. This is done by operating its various parameters, which include threshold, attack and release, oftentimes knee, and make-up gain.
The threshold determines at which point in the signal level the compressor will start to work. The attack time tells the compressor how quickly to react after the threshold has been surpassed, and the release time tells it when to let go after the the signal falls below the threshold again. Compressors with a knee setting allow the user to determine how “hard” or “soft” the attack is. And finally, make-up gain, or output, is a level control allowing you add back whatever gain has been reduced.
Compressors are extremely powerful tools used all over mixes, and sidechain compression is an additional tool used for making better sounding music.
What is a Sidechain?
Also called a “key” input, a sidechain is simply a separate input into the processor. It’s uses are varied, which we’ll start to dig into below.
Sidechain Compression Techniques
Sidechain Guitar to the Lead Vocal
This is a simple effect to set up. If you find yourself at the end of the song, and the lead guitar and vocal are both wailing away at the same time, this will help you mix them together. Send your vocal to a bus. Make that bus the key or sidechain input on a compressor that is on the lead guitar track. Now, when the guitar is playing and the vocals aren’t singing, the track will be out front. As soon as the vocal comes in, it will trigger the compressor on the guitar track and duck the guitar by an amount you set with the threshold.
Sidechain Overhead Mics to the Snare
This is another very useful sidechain compressor trick. If you record your overheads and you find that you have too much snare sound in them, you can simply set up a bus on the main snare channel. Send the bus to a compressor on the overheads. Now, every time the snare drum hits, it will trigger the compressor and duck the snare down in the overheads. When the snare is not playing, your cymbals will be uncompressed.
Create a Makeshift De-Esser
Before the days of de-essing plugins which are now incredibly handy, you could make a de-esser by sidechaining an EQ to a compressor. If you boost the highs on the EQ, those frequencies will be reduced when they hit the compressor’s threshold. Sibilance is really noticeable somewhere between 4 and 7 kHz, but you can also use a de-esser (or a makeshift, sidechained one) to tame some of the harshness of a vocal track. Sometimes setting a de-esser somewhere between 1 and 2 kHz can mellow out a harsh voice.
Sidechain a Rap Vocal to the Beat or Loop
In the same way that we’ve “ducked” the guitar and snare in the overheads with the previous tips, you can duck a loop or an instrumental for a vocal-heavy song, like a hip hop track. Insert a compressor on the loop that you want to duck and sidechain the main vocals to it. Now, every time the vocals come in, the loop will lower in volume. When the vocals exit, it will return to its original level.
This might be an interesting technique to try over beats with various elements which come and go. As the instrumental builds up and new pieces of the puzzle are introduced, sidechaining the vocals to a certain loop can help make way for not only the vocals themselves, but the new instrumental parts, too.
Sidechain the Kick to the Bass
This is common technique in most genres, whether you’re using a bass guitar and acoustic drums or kick samples and bass synths. When low-end heavy mix elements are fighting for space, it’s easy to give them a little help with sidechain compression. You start by putting a compressor on your bass track and keying its input with the signal from the kick.
Setting the right attack and release is crucial, because this will dictate how the bass ducks when the kick hits. You don’t want to be sucked so far down into the mix that it’s extremely audible. Rather, a subtle downward push to make way for the kick is preferable.
Sidechain a Sine Wave to a Kick
If you feel as though your kick drum is a little anemic, giving it a boost with a sine wave is a common technique.
You start by setting up a sine wave generator on a new track. Somewhere between 20 and 60 Hz is typical, but be weary of adding too much sub as you approach 20. You can also set the sine wave to the song’s key. After you’ve set up your signal generator, place a noise gate right after it.
Instead of using a compressor’s sidechain, you’ll now be using a gate’s sidechain. Key the input to your anemic kick, and you’ll be on your way to a meatier, low end monster. You’ll want to adjust the gate’s parameters so that the sine wave only sounds when the kick drum triggers it.
It never hurts to try some outlandish things when you’re mixing. The sidechain compression and gating techniques above are but a few ideas to get the ball rolling and the ideas flowing. Really, sidechaining can be as practical and utilitarian or as creative and weird as you’d like. As always, there are no hard and fast rules and no limits to what sounds great and what it takes to get there.
So, start experimenting with sidechain compression in your work!