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 improving an overall mix.

What is sidechain compression? Sidechain compression is a compression style/technique where the compressor acts to compress/attenuate an input signal/track according to the control of a separate signal/track (external sidechain) rather than the typical sidechain signal, which is a modified version of the input audio signal.

In this article, we'll discuss sidechain compression in great detail and consider how to get the most of the technique in our own audio productions.

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Table Of Contents

• What Is Dynamic Range Compression?
  • Compressor Parameters
  • Compressor Types
• What Is The Compressor Sidechain?
• How To Set Up Sidechain Compression
• Using Sidechain Compression In Audio Mixing/Production
• Related Questions

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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. 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, then, is an audio device (analog or digital) that produces this effect.

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:

• What constituted the loudest parts?
• By how much should the loudest parts be attenuated?

Compressor Parameters

Compressors will have threshold and ratio parameters (often adjustable) that answer these questions, respectively.

What is the threshold of a compressor? The threshold of a compressor is a set amplitude limit that dictates when the compressor will engage and disengage. As the input exceeds the threshold, the compressor kicks in (with its given attack time). As the input drops back down below the threshold, the compressor disengages (according to its release time).

What is the ratio of a compressor? The ratio of a compressor compares the number of decibels the input 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.

To learn more about compressor threshold and ratio controls, check out the following My New Microphone articles, respectively:
• Dynamic Range Compression: What Is The Threshold Control?
• Dynamic Range Compression: What Is The Ratio Control?

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 input 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 input signal drops below the threshold.
• Knee: the transition point around the threshold of the compressor where the output becomes attenuated versus the input.
• 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).

Compressor Types

Regardless of type, a compressor will have a gain reduction circuit that will effectively compress the incoming audio signal according to the aforementioned parameters. All compressors will also have a control signal or “sidechain” that effectively controls how the gain reduction circuit compresses the audio.

A compressor's “type” is largely defined by its gain reduction circuit. The typical types of compressors (with links to more detailed articles) are as follows:

• Variable-Mu (Tube): the gain reduction circuit is based around a remote cut-off vacuum tube.
• FET (Field-Effect Transistor): the gain reduction circuit is based around a field-effect transistor.
• Optical: the gain reduction circuit is based around an optical photocell assembly.
• VCA (Voltage-Controlled Amplifier): the gain reduction circuit is based around a voltage-controlled amplifier.
• Diode Bridge: the gain reduction circuit is based around a diode bridge circuit.
• Pulse Width Modulation: the gain reduction circuit is based around a pulse width modulator.

For more information on compression in general, check out my in-depth articles:
• The Complete Guide To Audio Compression & Compressors
• Top 11 Best Compression Tips For Mixing (Overall)

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What Is The Compressor Sidechain?

The sidechain, which effectively controls the gain reduction within the aforementioned gain reduction circuit, generally utilizes a peak or RMS detector or rectifier to read an incoming audio signal (AC) and convert it into a variable DC bias voltage for the gain reduction circuit to work with.

The sidechain signal path of a compressor converts an audio signal into a control signal for the compressor's gain reduction circuit. The sidechain tells the compressor when to compress the audio signal and by how much.

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

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).

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

As was discussed beforehand, “sidechain compression”, as a technique, is achieved with the latter. Note that the external sidechain path is independent of the feedback or the feedforward compressor signal flow design.

For more information on feedback and feed-forward compression, check out my article Feedback Vs. Feed-Forward Dynamic Range Compressors In Audio.

In the case of the side chain 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”.

So to recap, all compressors utilize a sidechain signal path. Sidechain compression, as a technique, is achieved by utilizing an external signal (other than the input audio signal) as the sidechain control signal for the compressor's gain reduction circuit.

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How To Set Up Sidechain Compression

Not all compressors will allow side-chaining.

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 (link to check the price at Perfect Circuit) is a Eurorack compressor module with sidechain capabilities via its Key input.

Warm Audio BUS-COMP

The Warm Audio BUS-COMP (link to check the price on Amazon) is a VCA hardware stereo compressor that features a dedicated external sidechain input at the rear of the unit.

Warm Audio is featured in the following My New Microphone articles:
• Top 11 Best Microphone Brands You’ve Likely Never Heard Of
• Top 11 Best Direct Inject Audio Box Brands On The Market

Smart Research C1LA

The Smart Research C1LA (link to check the price on Amazon) is an example of a 500 series stereo compressor with external sidechain inputs on its face.

For more information on 500 Series modules, check out my article What Is 500 Series Audio Equipment & Is It Worth It?

KiloHearts Compressor Snapin

The KiloHearts Compressor Snapin (link to check the price at Plugin Boutique) 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.

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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 in point form:

• Intense EDM-style pumping/ducking (kick or another percussive element as the external sidechain compressing anything/everything else).
• Voiceover clarity over music (VO as the external sidechain over music track(s)).
• Vocal clarity within music mix (VO as the external sidechain over competing tracks).
• Subtle kick drum and bass glue (where the kick drum or bass guitar/synth/element acts to sidechain the other).
• Sidechain-compress a single band of a multiband compressor to focus on a specific range of frequencies.
• Add rhythmic movement to both rhythmic and non-rhythmic elements (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, etc.)

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:

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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:

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