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Audio Compression - How to Use a Compressor

December 2, 2020

What is the purpose of a compressor?

A compressors main job is to reduce the dynamic range of an audio track. It makes the loud parts quieter and the quiet parts louder. Turns out, when you process an audio signal to reduce it’s dynamic range, you end up with a much cleaner, crisper sound.

Ableton Live Compressor

Ratio

Ratio sets how aggressive a compressor is. A ratio of 1:1 will offer no compression, the input and output signals will be the same.

The image above shows a compression ratio of 2.00:1. This means that for every 2 decibels above the threshold we set, the audio signal will be reduced by 1dB. Therefore if the audio signal reaches 10dB above the threshold, the output signal will be reduced by 5dB.

Attack

Attack dictates how quickly the compressor will kick in. For percussive sounds most of the time you’ll want to preserve the attack of the audio (i.e, don’t add compression to the initial hit of the snare drum). Therefore you’ll want a longer attack. However, for vocals you might want to start compressing as soon as an input signal is triggered.

Release

Release sets how long it takes for the compressor to stop compressing after the input signal has dropped below the threshold. Therefore the longer the release, the longer the compressor is applied.

Threshold

Finally, we’ve mentioned it a bunch already, you might already understand what the threshold is, but just in case not, the threshold is the level at which the input audio signal needs to be before the compressor starts to work.

GR (Gain Reduction)

The Gain Reduction level in the Ableton Live Compressor is a visual indicator that shows you how much gain is being applied. It’s not the most useful indicator, but hey at least we’ve got one. It also comes in a few different views. The second view gives a better look at the knee, which we’ll mention in a sec.

Out (Output Level)

Compressing an audio signal can drastically change the volume, the output level lets you reign in, or increase the wet level.

Knee

How the compressor switches between the uncompressed audio signal, and the compressed one. A soft knee is a more gradual change, whereas a hard knee is more sudden.

Ableton Live Compressor Knee

Makeup Gain

The Makeup Gain button on the Ableton Live compressor automatically applies gain to make up for the dynamic differences. To be honest, I never turn this on as I prefer to change my gain manually.

Peak

The peak button is for compressing audio the way we’ve been describing so far. The audio is compressed by a ratio for each x dB it exceeds a threshold.

RMS (Route Mean Squared)

RMS compression is a topic of it’s own, but to sum up, when your compressor is in RMS mode, the compression will only kick in once in incoming audio signals average level is above the threshold for a sustained period.

Expand

The expand option is less common, but features on the Ableton Live compressor. It works as an upward expander, meaning instead of making the louder parts of a track quieter, it makes the parts of the audio that reach the threshold louder!

Dry/Wet Mix

Choose how much percentage of the incoming “Dry” signal you want to be affected by the compressor (wet signal)

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