For its fancy sounding name and esoteric roots, sidechain compression is an extremely simple studio technique both in principle and practice. What is sidechain compression? Basically, it is the same as ordinary audio compression except that it uses another audio source as its input, and then uses the resulting gain reduction on the destination audio track. The most common application of this method is in the ubiquitous ‘ducking’ or ‘pumping’ effect heard peppered throughout just about every dance track recorded in the last 15 years. This effect is achieved by using the kick drum as the sidechain input, which then sends the resulting gain reduction to, usually, a sustained sound on another track like a pad, chord, or noise loop.
Using sidechain compression is all about finding out how your particular setup supports it. Some plugins (most compressors) support sidechaining but you have to find a switch or two to flip before it is activated. First, there will be a button that your DAW uses to activate the sidechain (in Cubase its a rectangular shaped icon along the mute/solo/bypass controls at the top of the plugin window). And, sometimes, there will be another button in the plugin itself that requires you to set the input to external (for an external source sidechain like the kick drum). Once that is all setup, all you have to do is send the source signal to the destination. Once again, in Cubase, you use the FX send on your source track to send it to the plugin which should now be selectable as a destination for the FX send. Its probably a good idea at this point to set the FX send to ‘pre-fader’ (meaning it will send the signal as it is output from the plugin, ignoring the fader settings), thereby allowing you to ride the fader all the way down on the source signal so you can effectively mute the source entirely. It is also a good idea to duplicate the source track and leave one as purely your sidechain source which isn’t even audible since it is set to pre-fader as described above.
The Pumping Effect We All Know And Love
In order to actually hear the effect, you will have to properly set all of the common compression parameters on your destination compressor plugin. A good starting point here is what I like to call the “Extremes Principle” where basically you jack all of the settings up to see/hear what result they produce and then ride them down to a much more judicious level gradually. So, for example, begin by sliding the threshold all the way down so that compressor remains active with even the most subtle source signal. Then adjust the attack to 0ms so it kicks in right away, and set the release to a good medium setting like 80ms. Also, set the ratio to its highest setting so you get the most dramatic range possible. At this point you should be hearing a very strong pumping effect on your destination track. I usually tweak the attack and release controls first to get the right rhythmic flavor out of the pumping effect. The attack could be left at 0ms but sometimes I find its more interesting if the attack is delayed slightly at anywhere from 5ms to 40ms. Sometimes this adds a slightly percussive sounding click just as the compression kicks in, which can be a favorable effect at times. Next, I adjust the release to something more in time with the beat so that the compression sucks back just as the next beat comes in. The shorter the release the longer the suction effect and vice versa. Next, you can tame the ratio to make the whole effect less dramatic and possibly allow for the sound to still pop up underneath the source track (particularly useful when using sidechaining on a bassline to clear room under the kick). You can also reduce the threshold to give a little variation in the amplitude of the suction effect.
Transparent Sidechain Compression
There are many other uses for sidechain compression that you may not even realize that go beyond the most audible effect outlined above and so explicitly heard in dance music. In fact, it is very common for mixing engineers to use sidechain compression on the most driest (in terms of fx) sounding song simply in order to de-muddify and effectively EQ out the overlapping frequencies between the bass drum and bass instrument. The same principles apply, however the goal here is transparency; to not even realize the effect is being used. A few DB’s of gain reduction on the bass instrument track is all you will probably need to gain clarity to the kick drum while not audibly changing the appearance of the bass intrument. An even better idea, to achieve this result, is to use a multi-band compressor and apply the effect only on the bass frequency bandwidth for even greater transparency.
Creative Multiband Sidechain Compression With FX
Speaking of multiband sidechain compression, it is also very common to see a creative use of sidechaining in which, most commonly, only the bass or treble frequency range is being effected, creating an effect reminiscent of a modulated filter plugin but patterned in time with the beat of your track. To hear this effect in action, try taking a very distorted sounding bass with lots of high-end fizz and slap a multiband sidechained to the kick drum (slow dubsteppy kick patterns often work best here) with the sidechain only activating on the frequency range of, say, 2khz-20khz. The result is sort of a patterned muffling that really adds drama to an otherwise static sounding bass line.
As you can see sidechain compression has many uses and yet is as simple to understand as manually riding a fader knob. Since sidechaining is simply using a different sound source for your input than your destination track, there are many other effects that can, in principle, use sidechaining as a way to modulate the wetness of a given effect such as a reverb (kind of like gating), distortion, chorus, etc. If you are interested in getting creative with sidechain effects like this, simply create a 100% wet track (one with 100% reverb, chorus, etc) and apply the sidechain compression on only this track. The key is experimentation. With a little bit of tooling around, you may just develope your own signature sound and find your static sounding tracks come to life. Have at it!