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[Mixing] Compression

category: Music | course: Music Mixing | difficulty:

Compression is the first technique you apply to a track, if it is needed of course. The name comes from the fact that you’re compressing the dynamic range of an instrument.

The dynamic range of an instrument is the range between the note with the lowest volume and the one with the highest volume. Note that this has nothing to do with frequencies – only volume. While a great dynamic range adds a lot of variety, in a digitally recorded mix it usually doesn’t sound or work as well.

Why? Because there is a limit to your volume, and there are other tracks trying to also get some attention. If your instrument has a huge dynamic range, there’s not much room for you to increase or decrease the volume. Because if you do, you’ll soon enough hit the top or bottom of the volume slider, and if you go beyond that – your track will be filled with noise. On top of that, a subtle change in volume of perhaps 1 or 2 dB might make the difference between not hearing the instrument at all, and hearing it at exactly the right level.

When would I use it? Mostly with audio recorded with a microphone. If you are for example recording a guitar chord progression, you might start with lots of energy (and thus volume), but as you play on drop that spirit and play a bit softer. That is the moment you’d use compression to bring those volume levels closer together.

How to Apply Compression

Compression is often included as an effect or insert, which you can add to a track. Some DAWs have a special button for it, but those are still only shortcuts to it. It is an effect, because compression doesn’t change the original audio file. It just changes how it outputs its volume levels.

Compressions work from the top. You can set a volume level, and if anything gets higher than that, it will be cut off so that it is underneath that level. This doesn’t mean that everything will just be reduced to the exact same level. Every note is scaled down accordingly, so that there are still smooth transitions between them.

And then when you have compressed your track, its dynamic range is much smaller, and you can turn up or down the gain to get it at the level you want again.

The Different Options

The compression interface in every piece of software is different, but it always has these options:

  • Threshold: this is the max volume level I just talked about. Every note that goes beyond this is lowered and compressed.
  • Ratio: this determines how much those notes are lowered. A ratio of 1:1 means that it is lowered exactly the amount which it is too much (so it becomes the threshold’s volume level).
    • Between 2:1 and 5:1 is fine for most things.
    • Lower ratios give a warmer and thicker feeling to an instrument
  • Attack: How fast the notes will be lowered. When the compression detects a note that is too loud, it will bring it down to the desired level in the amount of time you specified here.
    • A higher attack rate will let the first note of an instrument or first word of a vocal punch through as it starts. This is sometimes what you want.
  • Release: How fast it turns the volume back up for notes that are below the threshold. Opposite of attack.
    • The regular settings are almost always fine, but you can play around with it to see what happens otherwise.
  • Gain: While this can also just be set from the track settings, it is often also included within the compression effect. Turning it up simply makes the whole compressed dynamic range louder.


Golden Rules of Compression

Never, ever, do a low threshold with a high ratio.

Your compression effects window should have something called a gain reduction meter (GR). As you play a track with compression on it, this should display how much the volume is reduced at that specific moment.

The rule here is: if this meter doesn’t return to 0 several times a bar (remember, four quarter notes), you’re using too much compression. If it doesn’t return to 0, you’re compressing every note in the dynamic range, and that is just plain wrong.

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