The Complete Guide To Sidechain Compression In Audio

Sidechain compression is a powerful and creative technique in audio mixing that can have both drastic and subtle effects in the mix.

In this article, I'll break down sidechain compression in greater detail, explaining the intricacies of the technique and the jargon associated with compressors and sidechain paths. I'll also share my insights regarding getting the most out of the sidechain compression technique.

---

What Is Dynamic Range Compression?

Before we talk about the technique of sidechain compression, let's first cover our bases and develop an understanding of dynamic range compression (typically referred to simply as “compression”).

As the name suggests, compression is the process of reducing the dynamic range of an audio signal. The dynamic range of audio is the difference between the highest and lowest amplitudes.

So, in other words, compression is the process of reducing the difference in amplitude between the highest and lowest points of the audio signal. A compressor is an audio device (analog or digital) that produces this effect.

A compressor will have both an audio path and a gain reduction/sidechain path. The sidechain signal will effectively control the gain reduction applied to the audio signal.

Since the noise floor is often the lowest point of a signal, compressors will typically work by attenuating only the loudest parts of the signal.

In order to attenuate the “loudest parts” of a signal, two questions must be answered:

1. What constituted the loudest parts?
2. By how much should the loudest parts be attenuated?

These questions are answered by the [often-adjustable] parameters of compressors, notably the threshold and ratio.

Compressor Threshold

The threshold of a compressor is the set amplitude limit that dictates when the compressor will engage and disengage. As the sidechain level exceeds the threshold, the compressor engages. As the sidechain drops back down below the threshold, the compressor disengages.

Compressor Ratio

The ratio of a compressor compares the number of decibels the sidechain signal is above the threshold to the number of decibels the output signal is above the threshold. In other words, it is the relative amount of attenuation the compressor will apply to the signal.

Other Compressor Parameters

Other compressor parameters worth mentioning are the following (I’ve added links to in-depth articles on each parameter):

• Attack Time: the amount of time it takes for a compressor to engage/react once the sidechain signal amplitude surpasses the threshold.
• Release Time: the amount of time it takes for the compressor to disengage (to stop attenuating the signal) once the sidechain signal drops below the threshold.
• Knee: the transition point around the threshold of the compressor where the output becomes attenuated versus the sidechain.
• Makeup Gain: the gain applied to the signal after the compression takes place (typically used to bring the peaks of the compressed signal up to the same level as the peaks pre-compression).

Note that I've strictly used the term “sidechain” when referencing the signal controlling the amount of compression. In most cases, the default sidechain signal will be taken directly from the compressor's input signal. However, as we'll see, this sidechain signal can be altered and even taken from a source other than the compressor's input signal!

---

What Is The Compressor Sidechain?

The sidechain of a compressor is the signal that effectively controls the gain reduction within the aforementioned gain reduction circuit. This AC audio signal is converted into a variable DC bias voltage (control signal) via a peak or RMS detector or rectifier for the gain reduction circuit to work with.

This control signal tells the compressor when to compress the audio signal and by how much.

In addition to rectification (turning the AC audio signal into a variable DC voltage), the level detection circuit will also manipulate the control signal to achieve the desired compression parameters (threshold, ratio, attack time, release time, knee).

Here’s a simple signal flow chart to express compressor sidechain (note that, in the following example, the compressor is “feedforward”):

I'll note here that certain compressor types (gain reduction circuit types) will have limited control over the aforementioned parameters and may require a feedback circuit.

Because I've mentioned feedforward and feedback several times now, I should probably show the difference. Here’s a simple signal flow chart to express compressor sidechain (with feedback design):

As we can see, there is a switch in both of the above diagrams. The AC signal of the sidechain is generally taken from one of two sources:

• The program/input audio signal.
  • Feedforward design: the level detection circuit may take the signal before the gain reduction circuit.
  • Feedback design: the level detection circuit may take the signal after the gain reduction circuit (feedback design).
• An external audio input (external sidechain).

Note that the external sidechain path is independent of the feedback or the feedforward compressor signal flow design.

Of course, we also have our plugin compressors, that don't actually have circuits. Rather, they are programmed with this same idea.

As was discussed beforehand, “sidechain compression”, as a technique, is achieved with the latter source option (using an external signal as the sidechain).

In the case of the sidechain compression technique, the sidechain signal is taken from an external source rather than from the input signal itself. That's it. That's the definition of “sidechain compression”.

---

How To Set Up Sidechain Compression

Not all compressors will allow an external sidechain to be used.

For those hardware compressors that do, a sidechain insert will allow a sidechain source to control the compression applied to the input signal.

There will generally be a sidechain dropdown menu in software compressor plugins that will allow you to choose the sidechain source.

Read the manual of your specific compressor, or take a look at the inputs or menus to figure out if it allows for external sidechaining.

Let's consider a few examples:

2HP Comp

2HP Comp is a Eurorack compressor module with sidechain capabilities via its Key input.

Warm Audio BUS-COMP

The Warm Audio BUS-COMP is a VCA hardware stereo compressor that features a dedicated external sidechain input at the rear of the unit.

Smart Research C1LA

The Smart Research C1LA is an example of a 500 Series stereo compressor with external sidechain inputs on its face.

KiloHearts Compressor Snapin

The KiloHearts Compressor Snapin is a super-simple compressor plugin with sidechain capabilities via a dropdown menu at the bottom of the window.

Once we've identified that our compressor can accept an external sidechain signal, we have to choose what signal to use.

Remember that the compressor will be reading the external sidechain and using that information to adjust the amount of compression on the audio signal. Therefore, it's wise to understand the overall levels and the dynamic content of the external sidechain signal.

---

Using Sidechain Compression In Audio Mixing/Production

There are practically infinite ways to use sidechain compression in audio mixing and production.

The most obvious use of sidechain compression has been made famous/infamous by electronic dance music. That is the aggressive sidechain compression of nearly all elements according to the kick. In this case, a bus of tracks (perhaps everything but the kick itself in some cases) is passed through a compressor, with the kick track acting as the external sidechain.

Let's use this example of a regular kick as the external sidechain signal to visualize the effect of sidechain compression:

Remember our discussion on manipulating the sidechain control signal path to alter compression parameters? Let's now consider a second illustration where the threshold is raised and the attack and release parameters are increased:

But this is only just the start of understanding the power of compression.

Other percussive elements can be used with varying compression parameters to achieve all sorts of interesting sidechain compression. We can even use less transient sidechain material to control the compression of other audio signals.

Another excellent use of sidechain compression is to use a voiceover track as the sidechain input on a compressor acting upon a music track. In this case, the music level will get reduced as the voiceover is played back, and as the voiceover stops, the music will come back to its original level.

Though utilizing an element/track within a mix as the sidechain is common, it's not necessary. We can choose to use a signal that is absent from the mix (whether it's taken from outside the mix or muted from the master output of the mix). This way, we can have sidechain compression without hearing the sidechain element (kick, VO or otherwise).

As was mentioned, the applications are virtually limitless with sidechain compression. Here are a few common uses worth investigating:

Deep Pumping With Sidechain Compression

Achieve intense EDM-style pumping/ducking using the kick or another percussive element as the external sidechain compressing anything/everything else. Sometimes it's worth using serial sidechain compression to get deeper pumping with appropriate time parameters (attack and release).

Vocal Clarity Over Music (Including Voiceover) With Sidechain Compression

Sidechain compression is sometimes used to duck down the music under the vocal to help the vocal pop in the mix.

This is common practice when mixing vioiceovers over music.

In both cases, we use the vocal/VO as the external sidechain over music track(s).

Kick Drum & Bass Glue With Sidechain Compression

Subtle kick drum and bass glue can be achieved with sidechain compression, where the kick drum or bass guitar/synth/element acts to sidechain the other.

Band-Specific Sidechain Compression

We can get specific with our sidechain compression by focusing on a single band of a multiband compressor to duck a specific range of frequencies.

Rhythmic Movement With Sidechain Compression

Add rhythmic movement to both rhythmic and non-rhythmic elements with the use of sidechain compression.

For example, we can compress the hats to the kick with a high threshold and long attack time to add extra dimension, compress the guitar to the snare to add movement, and whatever else we want to add subtle amplitude-based rhythm to.

Dynamic Time-Based Effects

I personally love using sidechain compression on my reverb and delay return channels with the dry source acting at the sidechain signal. This allows the dry source to sound more upfront and less drowned out by the reverb and/or delay when it's present, while allowing the time-based effect(s) to bloom when the source isn't present.

So whether we're after clarity, pumping effects, or added “breath” in the mix, sidechain compression is an invaluable tool in our mixing toolbox.

I have a video dedicated to using sidechain compression in the mix. Check it out below:

---

Related Questions

Should you compress every track in a mix? As a general rule, compression should be used with intent and, therefore, only be used on every track in the case that every track would require it. More often than not, there will be certain tracks in a mix that sound perfectly fine (and better) without dynamic range compression.

Once again, the typical benefits of using compression on a track include (but are not limited to) the following:

• Maintaining a more consistent level across the entirety of the audio signal/track
• Preventing overloading/clipping
• Sidechaining elements together
• Enhancing sustain
• Enhancing transients
• Adding “movement” to a signal
• Adding depth to a mix
• Uncovering nuanced information in an audio signal
• De-essing
• “Gluing” a mix together (making it more cohesive)

What is parallel compression? Parallel compression (also known as New York or Manhattan compression) is a technique where one audio track (or several) is sent to a bus, and that bus is heavily compressed. Both versions of the audio are then mixed together to achieve a punchy sound without losing the dynamic of the dry signal(s).

I have a video dedicated to parallel compression. Check it out below: