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The Essentials Of Compression & Using Compressors

Answering the key questions along with some general guidelines on usage...


Some of the most common questions I receive about audio:

What is a compressor?

How do I use it?

What do the controls do?

Compression is like a finger on a volume control (fader). If a sound gets too loud, it’ll turn the volume down. If the sound is softer, it can bring up the volume.

You can also tell it to wait a few moments before it makes an adjustment, and you can tell it to wait for a little bit after the sound passes before the volume will go back where it was before. In a nutshell, this is what the controls on a compressor do.

Here’s a breakdown of each control on a compressor, and what it does:

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Threshold: Tells the compressor at what audio (decibel) level to start compressing at. Meaning, if you tell it to start compressing at -6 dB, then when the volume of an instrument hits -6 dB, the signal will start to be compressed when it reaches that level.

Ratio: Works together with threshold. Ratio is the amount of audio signal allowed to go above the chosen threshold. For example, if your ratio is set to 2:1, flip 2:1 upside down and turn it into a fraction = 1/2. So if the threshold is set to -6 dB and the ratio is set to 2:1, when the audio signal gets up to -6 dB, the compressor will only allow 1/2 of the original signal above the threshold.

If the ratio is set to 4:1, it will only allow 1/4 of the signal, and if it’s set to 6:1, only 1/6 of the signal will be allowed over the threshold, and so on. If the compressor is set to infinity, then it has been turned into a limiter and won’t let any sound above the threshold. Check out this handy dandy Compression Ratio Chart for reference:

(click to enlarge)


Attack: Tells the compressor how fast (or slow) to compress the signal once it hits the threshold. This is useful if you want to maintain some of the “punch” and dynamics of a signal, but still want to control its volume.

Let’s say that you have a kick drum that you want to preserve its initial transient (the first part of the hit)—you can dial the attack to start a little bit after the start of the drum hit happens, and then compress from there. This will in turn let the initial hit of the drum pass through uncompressed, and then start compressing at the designated time you’ve set the attack to.

For example, if you set the attack to 20 ms (milliseconds), the compressor will wait 20 ms after the signal reaches the threshold, and then once that 20 ms has passed, it will enact the compression.

Release: Tells the compressor how fast (or slow) to end the compression after it no longer reaches the threshold. So if you set the release to 150 ms, it means that the compression will end or “release” 150 ms after the signal goes below the threshold.

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