Side-chain Expansion - Bass and Kick processing

An advanced tutorial on how to use the side-chain of an expander to manage the kick and bass in a mix. The expander is used to extend the dynamic range as opposed to narrowing it like a compressor.

Beat Construction


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This tutorial is a tangential take on the trusted technique of using one sound to duck another sound BUT this time we are using one sound to expand (increase) the dynamic range of another sound.

The idea is to use the side-chain feature of an expander, that is placed at the keyboard channel of the DAW, and to trigger the side-chain using a kick drum. The expander increases the dynamic range of the keyboard line each time the kick drum plays giving a wonderful bouncy type of texture to the keyboard sound. In effect we are using side-chain triggering to increase dynamic range as opposed to narrowing it via the usual ducking processes we see on traditional downward compressors.

But before we get jiggy with this wonderful process let us take a look at both side-chaining and expansion.

Side-chain Expansion

The process is quite simple and something I am sure you have come across when it comes to managing the kick and bass in a mix. Quite often the bass and kick will be fighting each other for space, so we use one to duck the other. Every time the kick plays the bass is ducked slightly. This allows the kick and bass to sit together in the same space. We achieve this by placing a compressor on the bass channel’s insert. We then activate the compressor’s side-chain and use the kick to trigger the side-chain. We shape the behaviour of the compressor using its built-in features making sure to correctly set the threshold, attack and release values.

In this tutorial I am using the wonderful FabFilter Pro MB as it allows for expansion. The set up and execution of the expander’s side-chain is identical to that of a compressor albeit with an expander and instead of treating a bass sound we are extending the dynamic range of a keyboard sound and we are doing it rhythmically!

You can take this process a step further and use the side-chain filtering on the compressor to turn it into a frequency conscious trigger. In other words we can actually select a frequency range of the kick drum to trigger the expansion process.

Side-chain filtering

The part of the circuitry that monitors the input level is known as the side-chain, and it controls that part of the circuitry that adjusts the gain of the main signal path. Side-chains have both internal and external filtering. You can use filtering to specify which frequencies you want to filter from the circuit. A good example of this is to hi pass filter kick drums at 100 Hz. This means that anything below 100 Hz is ignored when activating the compressor. This is a great way to avoid clutter in kick compression. Additionally, we can specify a frequency range to act as the trigger. In effect, any frequencies that lie above and below this specified range are filtered and therefore ignored.

In this tutorial we are adding movement to a keyboard line and only having it triggered by a frequency range specified by the side-chain’s filtering feature.

Upward and Downward Expansion

There are two basic forms of Expansion we use in music production – Upward and Downward Expansion, and the best way to explain them is to compare them to existing processes like a gate and a compressor. Whereas compressors narrow the dynamic range of a signal expansion does the exact opposite and extends the dynamic range. A downward compressor will reduce the audio signal above the threshold whereas an upward compressor will boost the audio signal below the threshold. Both narrow the dynamic range of the audio signal.

Downward expanders reduce the level of an audio signal below the threshold, making quiet sounds quieter. This extends the dynamic range of the audio signal.

Upward expanders boost the level of an audio signal above the threshold, making loud sounds even louder. This extends the dynamic range of the audio signal.

In the video I generate a simple kick drum sequence and send it to the side-chain of an expander that is placed on the keyboard sound’s channel. I explain the various forms of expansion using the wonderful FabFilter Pro MB compressor and show you how different settings create different rhythmic textures. I show you how to use the expander’s side-chain filtering to specify which frequencies from the kick drum sequence trigger the side-chain.

Plugin used in this video:

FabFilter Pro MB

Topics covered in this video are:

  • Understanding Upward and Downward Expansion
  • Side-chaining Tricks and Procedures
  • Creative uses
  • Understand Threshold and Release
  • Frequency Conscious Bounce
  • Pumping Inversely
  • S/C Filtering