What is Compression and Limiting?

Compression is the reduction of the difference between the loudest and quietest parts of a given piece of music.

It is often used to make instruments sit better in the mix by lowering their overall dynamic range.

Essentially it is an automatic volume knob, which turns down the volume by a given amount.

Limiting is just more extreme Compression (10:1 ratio and above). Generally used to turn the volume down on occasional peaks by a large amount in one go.

One problem with compressors is that any peak in any frequency can trigger the compressor and lower or raise the volume for the whole frequency. Multi-band compressors get around this, by splitting the signal into multiple frequency bands (using filters). Compression can then be applied to each of the frequencies independently. They can be fiddly to set up though.

Setting Explanation

Threshold

The level at which the compressor will start working

Attack

How fast the compressor will take to react to the signal going over the threshold.
Slow attacks mean early parts of the signal will pass through without change.

Release

Time it takes for the compressor to return back to normal. i.e. stops altering the sound.

Ratio

The amount of compression once the signal crosses the threshold, i.e. how much to turn the volume down.
e.g. 2:1 = for every 2 Db over the threshold you'll get 1 Db of gain back out

Knee

Is the gradient at which the compression is applied. Denoted by Hard and Soft. Soft knee compression is when the compression gradually fades in. Often used for extreme \ limiting compression.

Hard Knee is often used on lower ratios, or for "brick Wall" limiting.

Knees

Most compressors come also with a Gain knob, to make up for the loss of level that compressors give.

Basic Settings

Th following is just a starting point. It all depends on the instruments used and how how they were recorded. I generally, start with these settings then tweak them. Print out this page and stick the table up on the wall for reference.

At the end of the day there is no magical settings.

Instrument Attack (ms) Release (ms) Threshold (Db) Ratio Knee

Drums 1 to 5 10 or auto -15 5:1 to 8:1 Hard
Bass Drums 30 - 40 20 ? 6:1 Hard
Snare Drum 30 - 40 5 - 10 ? 10:1 Hard
Bass 4 to 10 10 -2 to -10 4:1 to 12:1 Hard
Vocals Fast 0 or Auto -3 to -8 (try -5.5) 4:1 to 12:1 (try 6:1) Soft
Spoken Word Vocals 0.1 100 -1 2:1 to 6:1 Soft
Guitars Fast 0 or Auto -10 to -14 5:1 or 9:1 Hard
Acoustic Guitars 4 - 15 0 or Auto -14 5:1 to 9:1 Soft/Hard
Brass & Wind 2 - 5 10 or Auto -10 to -14 4:1 to 12:1 Hard or Soft
Overall Mix Fast 0 or Auto -5 to -9 2:1 to 5:1 Soft
Overall Mix 2 50 -2 to -5 0 2:1 to 5:1 Soft
Radio Compression 18 3 +6 4:1
Slow Gate 315 8.2 -2 5:1 Soft

General Tips

1.

Listen to the item you are compressing, with and without compression, preferably at the same volume, to see if it is really required. It may be better just to limit the peaks than compress the whole thing.

2.

Put the compressor first in any chain of effects. It gives the other effects a better signal to work with and won't amplify any noise introduced by those effects.

3.

Everything will benefit from a bit of compression on them.

4.

On percussive sounds use a longer attack time, so most of the sound passes through before the compressor kicks in. This should give the sound a more pronounced attack.

5.

Setting a fast attack and release time can give a pumping effect. (Used a lot in dance music)

6.

Female vocals allegedly require more compression than male vocals?

7.

Kate Bush uses two compressors on her voice. One to highly compress (Urei 1176) the other to catch any peaks that have got through.

8.

Compressors with auto release can be useful to maintain the dynamics of a track.

9.

Drums. For a distorted U2 type sound. Set a compressor threshold to -50dB and a ratio of 20:1 with fast attack and release.

10.

Sub group your drums to a stereo channel, apply stereo compression to get the drums pumping.

11.

As abaove but add some modulation effect (flanger, filters, phaser etc) for a bizarre effect.

12.

Vocals. For an etheral effect apply extreme compression to the vocals, then filter or EQ out the bass and most of the midrange (below 1kHz).

13.

Multi band compression on vocals can be useful to change the timbre of the vocals, e.g. more compression in the high end to give a vocal a sizzle, or to accentuatte a raspy voice.

14.

Multi band is useful, since a lot of the compression occurs from lower frequency noices. Giving the lower frequencies more compression can give a tighter louder mix. Alternatively, I suppose just up the compression on low frequency instruments?

15.

As with all effects check the sound with and without the effect to ensure you are not making things worse.

16.

To EQ then Compress or Compress then EQ. Try both, since you will get different effects.

17.

Compressors can bring up the noise level, so gate the sound prior to compression.

18.

If your compressor has a side chain, you can use it to trigger instruments from drums. e.g. you can tighten up a bass part by triggering it from the bass drum.

19.

A similar effect can be used to trigger a gate on a distorted guitar triggered from a hi-hat.