How to use audio compressors

By Peter Richter - Brisbane Audio Services

Ask ten different sound guys (gals) how to use compression and you'll probably get 16 different answers, but here's what works for me anyway.
I usually use one compressor over FOH. (For the purpose of this article, let's say we're running mono FOH) and one in the insert point of a sub-group to which all the vocal channels are assigned.
I’ve written this assuming you have an analogue style front panel on the compressor, whereby a strip of LEDs shows you how much compression is happening.

Initial settings for the FOH compressor:
Thresh-hold - low (least compression)
Compression ratio - around 4:1
Attack time - medium fast
Release time - medium fast

Initial settings for the Vocal Group compressor:
Thresh-hold - low (least compression)
Compression ratio - around 8:1
Attack time - fast
Release time - fast
I usually turn up the input and output levels a bit so that the vocals have a bit more level available over the rest of the band

Plug a mic straight into the desk on one of the vocal channels and set the gain as you normally would.
Turn up the channel fader and the group fader for the vocals to normal (say 0dB) and turn the FOH fader right down.
Speak / sing / yell into the mic.
Set the threshold on the vocal group compressor so that at a normal singing level, the first LED is coming on. (The LEDs that indicate how much compression is taking place).
Check that when you yell into it, that all but the last LED is coming on.
If you have the threshold set correctly (one LED on at reasonable level singing), but the rest are not coming on when yelling then turn up the compression ratio.
If they are all coming on as soon as you get above normal singing level, turn the compression ratio down.
That should be a pretty good starting point so that you can concentrate more on the FOH compressor when you sound check the whole band.

Get the band playing and set your levels to where you'll want them as per your normal sound-check routine.
Whilst doing so - keep an eye on the FOH compressor to make sure it's not "sucking" out all the volume as you are trying to bring everything up.
Once you are happy with the over-all levels, Go back and check the amount of compression happening on the vocal group compressor and check that normal levels are just bringing on one LED and that very loud vocals are bringing most of the LEDs on. Try and get all the singers to sing at once.
Turn up the FOH threshold until the main beat of the music is just turning on the first LED and that loud vocals are bringing on a few more of them.
That should be a pretty good setting then.
Make sure that you monitor the compression as you turn things up and down.
There are cases where you will want to keep the compression settings as they are to make sure this is the maximum level - especially where there are sound restrictions in place. There will also be cases where you will need to adjust the threshold as the night goes on and you want to increase the over-all volume.

Remember that when you are using compression, that you are "bouncing" all the different sounds against that threshold. You may find that if you can't hear a particular voice or instrument very well, even though you try to turn it up, that you might actually need to turn something else down to allow space within that threshold.

Sometimes, if there is just one vocalist - especially when they tend to sing very loud and very soft, without good mic technique, I put the compressor in the channel insert, rather than the group insert. This helps to prevent channel overload and also compresses what is coming through the foldback (to protect the foldback speaker).
If you do use the compressor on the channel insert - you need to re-check the foldback level as you make adjustments to the compression.
Generally though, I would not put compression on what the performer is hearing in the foldback, as it would reduce their need for good mic technique and you have more of a battle on your hands. (Although limiters would still be good for speaker protection purposes)

The LEDs are a good guide to see what is going on and to make sure you are in the window of normal operation, but remember, it’s all about how it sounds – so make sure you let your ears do the final decision. Practice, even with recorded music, to get used to what compression sounds like with different ratios and attack / release times.

Getting that punchy kick drum sound is often a fine balance of threshold, ratio, and attack time.

When using the sub-group method, try getting the effect return to go through that sub-group as well. You might find that certain feature instruments work well when fed through this sub-group too, such as sax.