Some plug-ins, such as Neutron 2’s Compressor, allow you to sidechain from a specific, definable frequency band (see image below).
Neutron 2 Advanced Compressor
Once you’ve got the routing and standard configuration taken care of, you can delve into some routine uses for this nifty process.
Typically, sidechains will only be found on dynamics processors such as compressors, gates, and dynamic EQs (though you may find exceptions). So, focus on threshold-based audio effects. To get started, here are some frequently-encountered situations that can benefit from sidechain processing.
Bass Drowning Out the Kick: Using the configuration detailed in the previous section, the bass will be compressed each time the kick hits. As a result, the kick will be more audible than the bass during those hits. This can be a great way to get the kick to punch through a bit more, especially if your kick and bass share the same frequency range.
Drum Overheads with Overwhelming Snare: Toss a compressor on the overheads, then send the snare (close mic) to its sidechain. Every time the close-miked snare is played, the snare-heavy overheads will be attenuated. If there is significant kick bleed in the close snare mic signal, use a high-pass filter on the sidechain to minimize it!
Bright Guitars Covering Up the Lead Vocal: Insert a dynamic EQ or multiband compressor on the offending guitars. Set it to subtly turn down or compress the frequency range where the vocal is present and bright. You know what’s next; send the vocal to the sidechain! Whenever the vocal is heard, the guitars will get just a little bit “mellower.”
Lots of Bleed in the Kick Out: Put a gate on the “Kick Out” track. Assuming that the “Kick In” track doesn’t have excessive bleed, send it to the gate’s sidechain. Now, the kick track without bleed controls the gate on the bleed-heavy kick. The bleed in the “Kick Out” won’t open the gate!
Fiending for Some ‘80s Snare Reverb: Set up a reverb plug-in on a spare channel. Insert a gate directly after the reverb plug-in. Send the snare track to the reverb channel and to the sidechain of the gate. Ho ho ho, dear readers. What happens as a result is not for seekers of the subtle. Each time the snare plays, the gate will open and let the reverb be heard, and the gate will close after each crack of the snare. So, the existing of snare signal triggers the reverb, and the lack of snare signal stops the reverb (abruptly or gently, depending on the gate settings). Wild and nostalgic!
As long as you have sidechain-capable processors and an unfaltering faith in the power of signal well-sent, the fruits of the sidechain tree are yours for the taking. Pull up some tracks and enjoy! You can also use sidechaining for some pretty creative mix effects, but that’s another story to be told some other time.