Learn Compression In Less Than 10 Minutes!

Do you keep browsing through compressor presets of which none fit your desires? Then it's time to take a few minutes to learn what a compressor really is does - and how to use it. Trust me, it's not that hard.

Before we start: two audio examples.

What is it?

A compressor is a plugin or a piece of hardware that reduces the dynamic range of the audio signal. This means that that the difference in amplitude between the loudest sounds and the softest sounds gets smaller. You can compare a compressor with an automatic volume knob, which lowers the amplitude of the signal when it exceeds a treshold. E.g.: Every time the signal is higher than -3 dB, the compressor turns the volume knob down a bit. Some compressors can also change the character of a sound to be more powerful, aggressive, broader, etc., but I'll leave this out in this article.

Why do we use it?

Because our ears are quite insensitive for dynamics, we can cut the peaks of the audio signal off and then increase the gain to make up for the cut-off peaks. The signal will be still peaking at the same level, but we will think the sound is louder. This is why compression is commonly used in electronic music, where producers want their tracks to be as loud as (if not louder than) the rest.

The original use of a compressor is to make the mixing process easier. It can be used to equal the vocal level so it can be heard anywhere in the mix (nobody sings perfect) or to give the drums a more powerful sound.

Concludingly, the amount of compressor used is not the same in every genre or style. Most classical recordings are barely compressed, whilst electronic music and funk are compressed heavily.

How does it work?

A typical compressor consists of two components: a level detection circuit and a gain reduction unit. First, the signal enters the level detection circuit. This component analyses the level of the sound: is it higher or lower than the treshold? If it's higher than the treshold, the level detection circuit sends a message to the gain reduction unit stating 'The signal exceded the treshold.' (Well not literally, in fact it's just a binaral 1.)

The gain reduction unit responses to this message by lowering the amplitude of the sound by a set ratio.

What can I change?

You can find the following controls on a compressor. Some compressors have other options, like specific algorhythms, and on other ones some settings are fixed.

- Input gain. Simply allows you to change the volume before it enters the compressor.

- Threshold (in dB). If the volume of the input signal exceeds this value, the sound will be lowered in volume.

- Ratio sets the amount of compression that takes place. A ratio of 1:1 means no compression. A ratio of 4:1 means that the input signal has to increase with 4 dB to have an increase on the output channel with 1 dB. Usual values are between 2:1 and 8:1. We call a compressor with a ratio of ∞:1 a brickwall limiter, although practically a ratio of 10:1 is the same.

- Attack (in milliseconds) sets the time it takes before the sound is fully compressed. E.g. with an attack of 20, it takes twenty milliseconds for the gain reduction unit to lower the level with the desired ratio.

- Release or Decay (in milliseconds) is the opposite of attack. It sets the time it takes for the gain reduction unit to go back to neutral after the signal came under the treshold.

- Knee specifies whether the compression comes abrupt (hard) or smoothly (soft).

- Make-up gain (in dB) is nothing more than a gain button to compensate for the loss of level in the compression process.

Now you know the basics of compression, be so kind to share it with your producer friends. In the future, we'll discuss several other applications and features of a compressor, like sidechain compression and look-ahead. If you want to get notified on every new article and thus the follow up of this article, like us on Facebook, follow us on Twitter and subscribe to our newsletter (top of page).

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