Sidechain Compression: What It Is And Three Ways You Can Use It In Your Mixes
Reading Time: 10-12 minutes
Hey guys! Let's chat sidechain compression, an extremely useful technique for getting the most of your mixes. We're going to start by defining exactly what it is, and then we'll talk about three ways you can use it.
WHAT IS SIDECHAIN COMPRESSION?
Let's first talk compression. Compression is a method of managing volume in audio, and it can also be used for creative applications. In general, a compressor will look at the amplitude of a waveform, and when it exceeds a certain volume (threshold), it will reduce the volume according to a certain ratio. In application, let's say you have a dynamic instrument like a bass, and you have a compressor plugin on the bass track set to a ratio of 2:1 and a threshold of -20 dB. Now, let's say that the bass hits -16 dB, which is 4 dB over the threshold. The compressor will reduce that volume from -16 dB to -18 dB, which is reducing it by a ratio of 2:1, or from 4 dB to 2 dB over the threshold. There are further parameters such as attack and release time, input filtering and whatnot, but let's keep it simple for now.
In sidechain compression, the principles are all the same, however, the compressor analyzes the waveform amplitude on one track, and then uses that information to compress a different track. In application, let's say we have a compressor on the bass again, however it is sidechained with the kick drum. In this case, whenever the kick drum exceeds a certain volume, the bass guitar would be reduced in volume. Why would we do this? In this instance, we're trying to get the bass to move out of the way when the kick is hit, as they share similar frequencies. This allows the kick drum to shine through the mix without having to turn down the overall volume of the bass.
And that's basically it! Now let's talk about applications.
1) MAKE YOUR DANCE TRACKS PUMP
This one is pretty standard in the world of sidechain compression. I can pretty much guarantee that you have heard this effect if you have listened to any EDM or pop music in your lifetime. Or weren't born under a rock. So what does it sound like? You know when you hear a group of synthesizers pumping in a mix? They duck in volume as soon as you hear the kick drum. It feels like the entire song is pulsing on the offbeat of the kick drum. That's sidechain compression in one of it's most classic uses. It's a cornerstone technique in dance music.
So how do you this? Well, it's not too much different than the example above, and it can be done in a few different ways. The easiest is to take all the instruments other than the drums and send them to a single bus within your DAW. You can choose to include or ignore certain instruments if you like, but the principle is the same. Next, put a compressor on that bus that has the ability to do sidechain compression. Two great compressors for this include the Waves C1 and the Waves C6 for multiband sidechaining (which is a whole topic on its own). Send the kick drum track out to an internal bus of your choosing. Assign the sidechain compressors "key" input to the bus that the kick is being sent to. You'll want to set the attack on the compressor to fast so that the other instruments will immediately duck out of the way of the kick. Then you'll have to play with the timing of the release to get the other tracks to pop back up in the mix at the right time. This can take some work, but typically you'll want the other tracks to come back 1/8th note after the kick.
Now having said that, there are a few tools that do this job wonderfully without having to set up any actual sidechain compression. I suggest you understand the technique but quickly move to one of these plugins: Xfer Records LFO Tool and Cableguys VolumeShaper for a faster and more automated approach
2) COMPRESS DRUM ROOM MICS TO SNARE
This is a really neat trick for getting some movement and energy out of the kit, as well as an additional resource for handling any minor phase issues. So let's start with the note on phase. Whenever you use multiple microphones on any source, you will have to consider the phase. Phase is simply the relationship between two waveforms. Drums typically have anywhere from four microphones to 15+ microphones at once (depends how crazy you're feeling). So, you're going to have phase issues to minimize and manage. A dead giveaway that you're out of phase is when you hear the bottom end of a source dropout when you blend two mics. So for snare, sometimes the overheads or the room mics can be out of phase and cause the bottom end of the snare to disappear, removing some body from the sound. Your first step is to try flipping the phase of the snare or the rooms to see if you can get a better phase relationship. You can also try alignment techniques such as AutoAlign. But if you want to go one further and let the snare drum close mic have it's own space, sidechain compression can be your friend. The set up is similar to above, have the compressor on the room mic channel and keyed to a bus that the snare is being sent to. Quick attack and fairly quick release will allow the weight and transient of the snare drum to get through and then releasing the room sound after, which is a very similar idea to pre-delay on a reverb plugin. This will allow you to get the room mics higher in the mix if that's what you choose.
The second and probably more interesting thing you can do with this method is get some movement and pulse going on the drum kit as a whole. The setup would be very similar, but instead of a quick release, trying timing the release with the beat of the song. Maybe the release is the same length as an 8th note, and you get this rhythmic pulsing of the room sound when the snare hits. This is a creative use, and should be applied with care. Each situation will be different, so you will have to play around with different settings to get this working to taste.
3) SIDECHAIN A FILL USING A GHOST TRACK
This one is pretty useful as well. Let's say that you have a fill that you want to accentuate, but the rest of the instruments in the mix aren't really jiving too well with it. Very common with drums, but it could also be a fill on any other instrument if the context is right. Let's also assume that this fill is awesome on its own and we'd really like to keep it in the mix. What do we do?
Well, we output all the other tracks outside of the fil together into a single buss. Let's use the drum example and say that this is all other instruments besides drums. Next, you'll need to set up a ghost midi track where the compressor will live. Put an extremely low CPU synth plugin on this channel, and then the compressor next in the chain. Set the output to off so that this channel doesn't actually play back.
Next, you'll have to program the rhythm of the fill to MIDI. Make the notes follow the context of the instrument your trying to bring out. If it's drums, keep the notes shorter. If it's a distorted guitar part, maybe keep the notes longer. Either way, match it up the best you can.. Then, set the compressor on the instruments bus to follow the MIDI channel you created. This means that whenever there is a certain volume on the MIDI instrument, it will duck the instruments out of the way. This is why we program the rhythm of the fill, then the fill pop out to taste . How cool is that?? In this way, you get to customize the rhythm of the sidechaining effect, rather than following the typical ways.
And that's it! Please comment below if you have any other ideas, questions, or concerns. We'd love to hear how you do sidechain compression and what it does for your mix. Comment below!
ABOUT THE AUTHOR:
Brad is the owner/founder of Velveteen Audio. He produces with the duo Towers and plays in his own project called Optics.
