4. Sidechain Compression
Sidechain compression has become something of a dirty word in recent years, but it doesn’t necessarily have to imply heavily-pumped French house or in-your-face EDM.
In fact, the process has been around for many years, though it was more often referred to as ducking, where the level of one audio source is used to ‘duck’ (push down) the level of another. When hardware ruled the roost this meant plugging another cable into the rear of a compressor to disrupt its normal signal flow. In a DAW this usually entails telling the compressor where to get its sidechain (level detection) signal. Here’s how it’s done in Logic (the sidechain selection menu is on the top right of the plugin window):
And here’s how it’s done in Cubase (click the image to enlarge):
Using an external sidechain signal effectively overrides the compressor’s normal operating mode, analysing the external signal to determine the level of gain reduction instead of analysing the compressor’s own signal input.
Used subtly, with a kick drum feeding the sidechain of a compressor across the bass, you can usually dial up to 6dB of compression before noticing any serious ‘pumping’ side-effects when the kick is playing.
It’s important to get the attack and release times right. Attack should be as fast as possible, though clicking may occur at the fastest plugin settings. Release is trickier. The release is largely determined by the decay time of the kick and the bpm of the track, so it’s often best to try and get the release to track this.
The main objective here is to carve out space by reducing the level of bass notes temporarily whenever the kick hits. This doesn’t have to be obvious; the effect can be used very subtly so it’s almost imperceptible when listening to the full mix but still cleans up the bottom end significantly. ‘Pumping’ sidechain effects use higher levels of gain reduction so that the effect is more obvious to the listener.
If you want a subtler effect but you’re finding it hard to hear exactly what’s going on, simply lower the threshold temporarily to exaggerate the effect. This makes the effect more pronounced, allowing you to get the timing just right before readjusting the threshold to achieve a suitable level of gain reduction.
A versatile, though long-winded, alternative is to programme a short click to play in time with the kick drum:
The audio from this track can then be routed to the compressor’s sidechain, facilitating much faster release times if required – just make sure you don’t send it to the main outputs!