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Sidechain Compression

April 29, 2020 6 min read

Sidechain Compression

Compression is one of the most common tools used in modern music production, and has been in steady use for nearly a century. The use of it has evolved drastically, where it once was applied mostly as a tool for cleaning up mixes and mastering, it is now caked on and pushed to the extremes. The core function of a compressor is to narrow the range between the loudest and quietest parts of an audio signal. In essence, it makes the peaks of your tracks quieter, squashes the waveform, and makes it more even in the mix.

For example, in your compressor, you pick a volume level for your sound to exist within. In that range, it will not go above or below the volume level chosen. You can alter the Attack on the compressor, which determines how quickly the compressor does its job, and you can also adjust the Release, which determines how quickly the compressor lets off. Additionally, the Threshold determines how much compression is applied. Producers, mixing engineers, and mastering engineers use compression every day on every track, similar to EQ. It is one of those effects that is vital to the formula of modern music. 

Other knobs on your compressor plugin that you may be wondering about are Ratio, Peak / RMS, and Knee. 

  • Ratio refers to the intensity of the compression for signals above the threshold. It determines how much compression will kick in once the volume begins to reach beyond the threshold.
  • Peak / RMS are used to determine how the compressor responds to the loudness of the trigger signal.
  • Knee determines the onset of compression. A soft knee, for example, fades in the compression, where a hard knee is much more abrupt, almost like an on & off switch.

All of these knobs and terms might seem confusing at first, and truth be told, compression is often quite subtle, making it hard to tell the difference between which knob does what. This is why we recommend playing around with a compressor on a reference track that you’re used to hearing. Push the knobs all the way in each direction to get a feel for what they do. Even if you never push them that far in your own music, it’s helpful to hear firsthand for what your compressor can do, and how each knob assists its job.

What is sidechain compression?

This part of compression is becoming ubiquitous in modern production. Even if you don’t know what sidechaining means, you’ve definitely heard it in songs like “One More Time” by Daft Punk or “Tea Leaf Dancers” by Flying Lotus. The effect has soared in popularity because of its rhythmic effect and its powers as a mixing tool. In short, sidechain compression is a unique type of effect in which the compression level on one audio track is controlled by another audio track. 

One of the most common examples is using a sidechain compressor on a bass track and routing it to the output volume of the kick drum. This way, when the kick drum hits, the bass track is compressed, thus providing more room for the kick drum to fit in the mix. 

 

What is the purpose of sidechaining?

As a mixing tool, if you have multiple low-end instruments in your track, they are going to be fighting against each other to be heard in your mix. This is why in the example we mentioned above that you would sidechain your bass to your kick. If you have a sub-bass and a punchy kick, when the two hit simultaneously, it not only reduces the punch of your kick, but it adds mud to your mix because of the overwhelm of low end frequencies. But if your bass is compressed each time your kick hits, it makes room for the kick, allowing the low end frequencies not to be too crowded AKA muddy. 

It can also be used to help clear up room in your mix for vocals. If you have a singer with a soft voice and / or a busy instrumental, you may consider putting a compressor on a few of those more mix-occupying instruments and sidechaining them to your lead vocal. This way, whenever your singer is struggling to stand out amongst the busy mix, the sounds are compressed, leaving space for the vocals to shine clearly. You can apply this type of mixing principle to any sounds that you want to be compressed in order to make room for others. 

In more modern genres like Lo-Fi and Chillstep, sidechain compression is often used without an obvious mixing purpose; rather, it’s applied stylistically in order to add rhythmic variations to sounds. For example, you may have an ambient pad that you sidechain to your kick, making it so what would otherwise be a droning chord becomes a rhythmic tool to add dynamics to your beat. An example of this would be the Teebs’ song “View Point,” in which you can hear the ambient, string-like pad “ducking” each time the kick hits. In Lo-Fi, producers might put a sidechain compressor on a guitar track with a low pass EQ filter on it. They will apply an extreme amount of compression to the guitar, making it “duck” each time the kick hits, which you can hear on Quickly, Quickly’strack he composed for our Lo-Fi & Chill pack. However, the sounds aren’t “ducking,” they are being heavily compressed, thus making them appear quieter.

How do you use a sidechain compressor?

Most DAWs will have the sidechain feature built-in to their compressors, however, depending on which software you use, you may need to download a compressor that includes sidechaining. For this example, we are going to use Ableton, but the same concept applies to each DAW. Begin by selecting the track you wish to route your sidechain compressor to: in this case, we will be putting a compressor on a bass track, and routing it to a kick drum 

Once you select the kick drum, you will adjust the relevant knobs to determine how much compression you want applied to your bass when your kick hits. The main four to look out for are: threshold, ratio, attack, and release. As you adjust each knob, you will notice the difference in compression when the kick hits. 

 

For an exaggerated use of the effect, you might push your ratio to something drastic like 18:1, limit your attack to next to nothing, set a short release, and push your threshold far down to something like -30 dB.

 

In addition, in Ableton, you can select the EQ knob in order to put a filter on what you’re sidechaining to. This might come in handy if you have a drum loop with a kick, snare, and hi-hat in it, and you want to route your sidechain compressor to the kick, but since the kick is baked in with the snare and hi-hat, you can’t just select the kick. However, if you use the EQ option, you can select a filter that routes it to whatever frequency you set. In this scenario, that might be any frequency below 200 Hz, which will be where the kick is hitting hardest, and shouldn’t have much snare or hi-hat. You can also adjust the Gain and Mix signals, giving you even more room to dynamically tweak your compressor to sidechain exactly as you need. 

 

Is sidechain compression necessary?

In most cases, if you balance your mix well, you don't need much sidechain compression. The only instances where it might be deemed necessary is if you have clashing frequencies between two instruments like a boomy kick and an 808. There has to be room for those low frequencies to coexist, and if they’re hitting at the same time at similar volumes, the whole mix is going to become muddy, and the kick will fall flat. In a scenario like that, sidechain compression will fix the issue right up. 

On the other hand, using sidechain compression to create a rhythmic “ducking” sensation is purely a stylistic choice. There is no need to put a sidechain compressor on every track and route it to a kick or something. Though, if you’re looking to create in the Chillstep or Lo-Fi genres, you will likely be using the effect very, very often. In fact, there’s really no limit to what you can sidechain. For example, you could have an audio sample of vinyl static and sidechain it to a snare drum, giving the vinyl static track its own rhythmic movement. These little uses are great at adding variety to beats.

However, be careful not to go overboard. Oftentimes, less is more. You don’t need to hear your track “duck” in order to know you’ve used enough compression. As long as the mix is clean and the desired effect is achieved, you can stop there. It can wind up sounding a bit tacky if you are cranking the compression on every other track, even if it feels exciting in the moment. The key is adding just the right amount in order to get a clean and stimulating mix.



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