What Is Sidechain Compression?

Before we talk about the sidechain, it’s important to talk about compression. Compression is used heavily throughout the Dreaming City album. Taking inspiration from electronic music group Justice, compression is the glue of the album.

Limiting Compressors

Songs throughout the Dreaming City contain many samples, ambient noises and trippy reverberations. These sounds aren’t necessarily cohesive when played together, but when pushed through the same compressors, the sounds begin to mesh. Two pieces of music or sound may mesh when their notes are in the same key, but also when their effected by processing in the same way.

With a Limiting Compressor, sound begins compressing if the volume reaches a certain level, called the “Ceiling”. Everything past that level is heavily compressed, and is said to be “Limited”. The goal is to compress the sound so it’s not to go louder than the ceiling.

Compressor controller showing the compressor Ceiling set to -0.1dB and a Threshold of -9.7dB. True Peak limiting is enabled.
The limiting compressor used in the song Length of a Moment (Feat. Steven Clapham)

The other parameter is the “Threshold”, which enables gain compensation to boost sound that is quieter than the specified level. This shifts all sounds into a loudness zone, where nothing goes quieter than the threshold and nothing louder than the ceiling.

Sidechain Compression in the Dreaming City

Overall, sounds are all filtered through the master limiting compressor, but individual elements are first affected by their own compression. In this album, all bass heavy synths have sidechain compression applied, to let the drums push through the mix.

Sidechain compression takes one sound as an input and one sound as the effector. The input sound decides how much compression will be applied to the affected sound. In my mix, the input is almost always the kick drum. When the kick drum hits, the compression starts, and the affected bass synth is dampened, allowing the kick to sound large and full of bass.

Sidechain compression in Length of a Moment (Feat. Steven Clapham)

In the preview above, the top module is listening to the kick drum, displaying it’s volume over time. Below is the compressor, affecting the bass synth. On the right side of the compressor, you can see the gain knob ducking in sync with the kick hitting. Listening to the bass, you can hear how the sound seems to pulse with the kick, when usually the bass line would sound like a constant tone.

This form of compression applied to the bass synth is constant across it’s audio level, unlike most compression. This means no matter how loud the bass is, it’s signal will be lowered by the kick drum. If the bass was more dynamic, and included quieter elements, along with louder elements, that worked with the kick, then a more typical compression would be used. to keep the dynamics of a softer sound, but dampen the louder sounds, the side chain would effect the threshold knob instead of the gain knob. Additionally, the ratio would need to be set to something like 10:1 so the sound within the threshold would be 10 times quieter.