Compression

Quick 'n dirty version - A compressor is a little man in a box with a volume control.  If you get too loud, he turns you down.  When you're level goes back to normal, he turns you back up.  The big advantage is that he does this faster than any human being could possibly do it by hand.  It basically keeps things even.  It also protects your system from loud spikes in volume like someone screaming into a mic unexpectedly or maybe someone dropping a live mic, etc.

He draws a line called the threshold.  He can draw this line to affect only high volumes, or low and high volumes both.  Once you cross it, he can turn you down a little or a lot depending on how you've set him up.  The amount of, "a little or a lot" depends on your ratio control knob.  With a high ratio, he'll turn you down a lot.  With a low ratio, he'll turn you down a little.

dbx is a company that makes great compressors.  They have a wonderful document called Compression 101.  We have included it here for your reference.  It is completely authored by the engineers at dbx and they did a great job.  Enjoy.

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Brian here from Media Write Group. Personally I believe in a live situation (in general), 12dB of gain reduction on a vocal or vocal group, is just plain too much.  In the studio it's different, but live... too much compression can lead to feedback, as can too much reverb.  My personal approach to compression and live vocals is as follows:

*If you're compressing a subgroup, skip step (1).

(1) Set the gain on your channel with nothing inserted, because once you insert a compressor, it will affect what your PFL meter tells you.  It will LIE to you!!  So, set your gain on the vocal channel with no insert (physically pull the 1/4" connector out while setting gain).  This insures that you are not overdriving or distorting your mic preamp.  Then, with that done, insert your compressor.  If you're compressing a subgroup, ignore what I just said because there is no mic preamp on your subgroup.

(2) Turn the Threshold knob all the way up (just for now), and set your Output Gain knob to Zero (12 o'clock).  This insures that you are NOT compressing anything (threshold), and you are NOT boosting anything (output gain).

(3) Set your ratio knob to 3:1 or 4:1.  4:1 is more compression.

(4) Have your people start singing at performance volume (not sound check volume).  Have them play a song with the band if need be. Assume they will "sandbag" you and get louder later. Plan ahead for this. :-)

(5) With your talent singing, turn down your Threshold knob until, at their loudest, they are getting about 4dB - 10dB of "gain reduction" on the meter.  You'll usually have to dial this in a bit more during the show because show volume is always a bit different.  If they sing softly, my personal belief is that there shouldn't be any compression at all.  When they sing loudly, there should be around 6dB or more of gain reduction. Loud things on stage like drums, etc. can trigger your compressor before your vocal will. You'll just have to live with this. It's one of the things that makes live sound different from studio sound.

*note*  Your talent needs to be right up ON the microphone, not standing a few inches away from it.  The farther away they are from the mic, the more you have to turn up the mic-preamp gain, and the more likely they are to feedback.  Have them get right on top of the mic.  I usually advise them to kiss it, or touch it gently during performance.  This is important for many reasons.  Just keep them ON the mic, and singing as loudly as they feel comfortable with.  Soft, timid, or quiet singers just make life more difficult (especially when they want their monitors louder).

(6) Now that you're compressing them you'll notice they have gotten quieter because of the compressor.  They don't call it "gain reduction" for nuthin'.  This is what the Output Gain knob is for.  Turn it up to boost the (now compressed) vocals and bring the volume back up to whatever you need.  Use as little Output Gain as you can because this extra volume will be there all the time; during quiet times, during loud times, and pretty much all the time.  If you notice you're always pushing this fader up higher than the others, bring it back down to normal, and turn up the Output Gain on the compressor.

(7) There are two basic compression schemes: Soft and Hard. Soft loosely translates to low threshold / low ratio. This means you compress things softer (~2:1), but start compressing them sooner (-10dB or lower). Hard translates to high threshold / high ratio. This means you set the threshold higher (0dB+), but once they cross it, you hit 'em hard (4:1+). Soft is generally better if you can get away with it because it's theoretically smoother. However, if someone screams suddenly, or drops a mic, your low ratio may not be enough to protect your system adequately. If your comp has an output limiter in addition to the compressor (like a dbx 166XL), you can get away with it. If your compressor does NOT have an output limiter (like a dbx 266XL), then you might be safer going with the "hard" scheme.

Hope this helps.