Most people working with audio will be familiar with gating, a gate turns down the signal completely once it falls below a set threshold. Gating is an extreme form of downward expansion, that’s why you’ll often see the tool being referred to as an expander/gate. In essence, an expander is a tool that can increase the dynamic range by either turning down a signal below the threshold (downwards expansion), or turning it up above a set threshold (upwards expansion). While expanders are widely used as noisegates, or creative effects with use of the sidechain input, subtler ratios can also be incredibly useful to shape the envelope of a sound. This article is specifically written to broaden your knowledge of this versatile tool, making it a powerful asset in your mixing arsenal.

Downward expansion

A downward expander allows you to turn the signal down by a set ratio once the signal drops below the threshold, gating being the most extreme form of downward expansion. Used with moderate ratios a downward expander can be a powerful tool to change the dynamics of an instrument or dynamic relationship between instruments. In most cases, you’ll use the downward expander to solve issues that revolve around the sustain, or tail of a sound.

Let’s consider you’re encountering some masking issues between a kick drum and a bass guitar. While you can achieve great results with eq, expansion can sometimes solve the issue without changing the tonal quality of an instrument. A kick drum with a long sustain can easily start to mask lower notes of a bass guitar which usually sustain longer than a kick drum. With a downward expander you can surgically tame the sustain of the kick so the bass notes can have some space to bloom. To do this, start with a hard knee to ensure you can be very precise and surgical with your threshold. Adjusting the attack and release controls allow you to carefully adjust the tail of the kick drum. A reasonably fast release time can significantly reduce the sustain of the drum, and if present the reverb/room bleed.

When you’re using this technique on low-end heavy instruments with a lot of sustain, the sustain can tend to keep the gate/expander open. If that’s the case, it is possible you’ll have a problem adjusting the threshold. Also, very fast release times can start to introduce distortion as the gate/expander starts to react to individual cycles of the wave.

In this case, you can use a sidechain filter to exclude the lower frequencies of the instrument. By only triggering on the attack of the instrument you’ll be able to control the timing of the low end more precisely.

Using this in a multiband context it creates a powerful tool to get rid of any recorded bleed, typically kick/snare/hi-hat.

Upward expansion

Unlike a compressor which turns the signal down above the threshold, a upward expander turns a signal up above the set threshold, which is why it can be thought of as the exact opposite of a traditional compressor. Upward expansion allows you to rescue some of the material that might have been compressed a bit too much, allowing you to restore some of the dynamics. Unfortunately, sources that have been compressed too much can’t be fully restored using this technique so it is best not to over compress in the first place.

Upwards expansion can have many other creative and/or problem solving applications however, especially when restricted to specific bands of frequencies.

Upward expansion can be great if you want to increase the attack of an instrument or want it to have more impact. A typical use would be to lift up the low end of a kickdrum or increase the attack on a (fingerpicked) acoustic guitar. It can also work great if you’re working a lot with sample libraries as programming with midi can often lack some excitement a real player adds to a part. Just a little bit upward expansion can have a lot of effect on the impact of a sound, just 1 dB of expansion can make something stand out in a mix, or make it more engaging to listen to.

Noise Gate

A noise gate is a downward expander in its most extreme form. Only sounds that will rise above a set threshold will allow sound to pass through i.e open the gate. Traditionally noise gates were primarily used in live applications getting rid of rumble or bleed for when an instrument wasn’t playing a part. However, besides solving problems and lowering the noise floor, a gate can also prove to be an incredibly creative tool. A trick that is often used is using percussive sounds to trigger a gate that is initiated on a sustained sound, e.g a synth pad.

If you want to turn your gate into a tremolo (as presumably used on the track “Money by Pink Floyd), you can use two sine wave generators (at a very low frequency) out of sync with one another (e.g one at 20 Hz and one at 24 Hz). You can use the beating it causes as a trigger for the sidechain input. Of course, you can also use a LFO at the sidechain input if you want to make it a bit easier. Unlike most tremolo’s which will just give you control over speed and depth you’ll have much more control over the character of the tremolo.