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Think of compression being like an invisible finger on a volume control (fader) and you're already halfway there...

Dear Casey,

I’ve been using our new digital mixing console for a while now, but I’m not sure how to use the compressor on it. Can you give me some practical tips on what it is and how to use it?

Sincerely,
Compression Newbie

Dear “Newbie” —

Let me explain what a compressor is in a very simplistic way: think of compression as being like an invisible finger on a volume control (fader), and you’re already halfway there. Now, for what every control on a compressor does, think of this analogy and apply it to the following:

• If a sound gets too loud, it’ll turn the volume down.
• If a sound is too soft, it can bring up the volume.
• It can wait a few moments before it makes an adjustment.
• It can wait for a little bit after the sound passes before the volume will go back where it was before.

This, in a very simplistic nutshell, is what the controls on a compressor do. Now, let’s go over each control on a compressor in more detail…

Threshold
The threshold tells the compressor at what decibel (dB) level to start compressing. In other words, if you tell it to start compressing at -6 dB, when the volume of an instrument hits -6 dB, the signal will start to be compressed at that point.

Ratio
Ratio and threshold work together. “Ratio” is the amount the audio signal may go above the chosen threshold. To determine this, follow this formula:

If the ratio is set to 2:1, invert the 2:1 and turn it into a fraction = 1/2 (one-half)

For example, 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 the ratio is set to 6:1, only 1/6 of the signal will be allowed over the threshold. If the compressor is set to infinity, then it has been turned into a limiter, and it won’t allow sound above the threshold.

Attack
The attack knob tells the compressor how quickly (or slowly) to reach full compression after the signal has reached 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 with kick drum, you want to preserve its initial transient (the first part of the hit) — dial the attack to start a little bit after the start of the drum hit happens and then compress from there. In turn, this will let the initial hit of the drum pass through uncompressed, and then start compressing at the designated time you have set the attack to. For example, if you set the attack to 20 milliseconds (ms), the compressor will start compressing the signal as soon as it hits the threshold and will reach full compression at 20 ms.