Com­pres­sion is a really sim­ple tool which is not hard to grasp at all, but most peo­ple strug­gle with, so if you don’t know how/why a com­pres­sor works, you’re not alone!
You just need to know these few things:
You have Thresh­old, Ratio, Attack, Release, Gain.

  • Thresh­old: Say you put a sam­ple in your com­pres­sor which peaks at –9dB, the thresh­old will deter­mine when the com­pres­sor should kick in. So if you set the thresh­old to any­thing higher than –9dB, it won’t do any­thing (since the drum will never be louder). If you for exam­ple set it to –10dB, then it WILL kick in at any peaks above –10dB.
  • Ratio: Now the ratio will deter­mine how much to lower the vol­ume. If you set your com­pres­sor to 2:1 it will turn down every 2dB that’s over your thresh­old to 1dB, this means that if it’s 4dB over the thresh­old, the com­pres­sor will make it 2dB and so on. With a com­pres­sion of 3:1 it would turn down every 3dB that’s over your thresh­old to 1dB and like­wise if it’s 6dB over the thresh­old, the com­pres­sor will make it 2dB and so on. In other words, ratio deter­mines how much it will squash of the sig­nal that’s louder than we allow (with the threshold).
  • Attack: The attack will deter­mine how much time should have passed where the sig­nal is over the thresh­old, until it com­presses the sig­nal accord­ing to the ratio.
  • Release: This will deter­mine how long it should com­press the sig­nal before let­ting the sig­nal be uncom­pressed again.
  • Gain: Gain will only let you change the vol­ume after com­pres­sion, so there’s noth­ing spe­cial about that.

As you can see, this is rel­a­tively straight­for­ward, but some peo­ple may be con­fused as to when and where to use com­pres­sion. The say­ing usu­ally goes “don’t com­press unless you need it,” but it’s non-trivial to know when you need it. A good tip is to apply it on things that should cut through the mix or should be snappy (like a snare for example).

A sim­ple way to see how com­pres­sion affects your sound is to do the following:

  • Set the thresh­old to as low as you can (like –80dB).
  • Set the ratio to as high as you can (approx 10:1).
  • Set the attack to 0ms and the release to max­i­mum (maybe 300ms).
  • Now you should only hear a small click on what­ever sam­ple you are using. This is a prime exam­ple of the com­pres­sor work­ing. Tweak your set­tings from here to see how much com­pres­sion you want.

Good things to bear in mind:

  • Gen­tle com­pres­sion can give a nice touch to things. In order to be gen­tle, have a fairly high thresh­old and a fairly low ratio.
  • The attack should be fairly short if you want it snappy. Less than 10ms usually.
  • The release should be adjusted to the speed of the song, so don’t leave it at maximum.
  • Com­pres­sion alters the tran­sients of your sounds, so be care­ful with what you compress.
  • If your com­pres­sor has auto­matic make-up gain, turn it off. The rea­son for this is that it will seem like the thresh­old will turn the vol­ume up, rather than down, because the com­pres­sor will make up for the vol­ume that’s lost to com­pres­sion, by gain­ing it. Only enable this if you know what you’re doing or else it will be counter-productive.
  • Don’t let the loud­ness fool you. Even though your com­pressed sig­nal will be qui­eter than the orig­i­nal (and there­for prob­a­bly sub­jec­tively sound worse), you should remem­ber that you can use the gain.
  • Hard com­pres­sion hurts! go easy on the ratio-meter.
  • Don’t over­com­press! It kills the tran­sients and causes lis­tener fatigue.

That’s all there is to it! Now go on and experiment!

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