What it is and how to start
What is a compressor?
Inside of a compressor is a little elf. A really quick dude with great ears … well, all elves are like that, aren’t they?
In the typical compressor, the elf does his job according to instructions that generally contain four numbers or “parameters”.
The elf listens to the input and has his hand on a volume knob. If the loudness goes above a limit (a parameter called the threshold), he turns down the volume on the output. The amount he turns it down is based on how high the loudness went above the threshold, and another parameter called the ratio. The bigger the difference between the loudness and the threshold, the more he turns it down. And the higher the ratio, the more he turns it down.
When he turns the knob down, he doesn’t do it in one quick click, but actually turns the knob in a smooth adjustment. The speed of this adjustment is governed by a parameter called the attack rate or time constant.
If the loudness goes back down, the elf turns the volume back up, but at a much slower rate, governed by the last parameter, the release rate or time constant.
So if the elf only lowers the volume, how does this make things louder (a common reason for using a compressor)?
Well, there’s usually a boost after the elf, sometimes called the post gain. It’s usually a constant boost so the elf doesn’t need to be concerned by it. So, by adjusting the amount of boost, you can control how much you’re raising the quietest parts (if at all) versus how much you’re lowering the loudest bits (if at all).
BTW, that’s just a typical compressor. There are other varieties, like opto-compressors. These were the first variety and worked something like the above but the effect was due to lucky physics when passing the sound optically through a light bulb and photoreceiver. Engineers noticed this effect and built components for the purpose. Also, there’s tape saturation, which also causes a very lovely sounding compression. And then there are special-purpose compressors that work like the typical one, but have parameters in different ranges (limiters), or that work in rather different ways (brickwall limiters). Finally, different compressors use slightly different control laws (math), leading to much confusion in deeply technical discussions with laymen like me. 
How should we set up a compressor?
Generally, the best way is to set the ratio to a given “starting” value like 3:1. Set the threshold to the top (0dB), and while listening to the material, draw the threshold down until you hear the compression working the way you want it to sound. Sometimes it’s a good idea to go a bit too far and then back off. The usual rule about avoiding over-use applies here, so it’s usually a good idea to set it how you like it and then back off a bit on principle.
Now you’ll find that the material is quieter than it was. (Unless the compressor you’re using has “automatic gain compensation”, in which case it will sound louder.) In any case, you can compensate for the loudness change using the compressor’s post gain. You want to adjust the post gain so that the peaks are as high as possible without clipping (as usual). Then you’ll need to adjust the track fader to make the part fit well in the mix.
If you didn’t achieve the amount of compression you need, set the ratio higher and try again. With experience you’ll get an idea of what ratios serve best for various purposes, and you’ll have a better idea what ratio to use for your first try.
Practical applications
While the purpose isn’t to increase the overall volume, it will tend to have that effect, at least a little. For starters, apply it on vocal, bass, and acoustic guitar tracks, and any other tracks where the volume varies more than you think it should – i.e., tracks that sound too loud in some parts and too quiet in others. Use compression to smooth this out.
When adjusting the compression on a track, make sure you’re listening to the whole mix, not just the track you’re adjusting. (Sure, you can listen to just the track to help understand what the controls do. Just don’t make any judgements about what sounds best unless you’re listening to the whole mix.) After you adjust each track, go back and do it again, because you’ve just changed the whole mix. Adjustments, if any, should get smaller on each pass. When adjusting the compression, you’ll also be adjusting the track fader because the compression affects the track level. (I.e., fuss with all the knobs!)
Then mix to get a good overall sound without worrying too much about overall volume. Make sure there’s no clipping in the mix. After applying some compression to individual tracks, though, you’ll find that the overall mix can be louder without clipping.
Cheers, Learjeff
Thx Jeff where ever you are! Other information about mastering by learjeff can be found HERE
I only got as far as the elf. Elfs (or elves for the spelling challenged) are tricky little folk, and I don’t want to have anything to do with them.
Cheers for that, Levi. That’s nicely put. Easy to grasp.
Oh, are you in league with the elfs, then?
Urh? Like shooting elves in a barrel.
I just tend to turn the knobs left and right on my Rane VP12 till my voice sounds good!
(sorry but some of the caps or missing off the nobs so I can’t always tell where they are pointing, hard right or hard left then back them off till it sounds balanced!)
Then there is the RANE DC 24 compressor crossover I send the whole mix through…again, just got to keep moving things around till it sounds fat out of the PA! lol
It sure would be nice to know a general setting or two…Learjeff is the man around here when it comes to that, I did try to read one or two of his suggestions a few years back (about vocals) but I quickly got lost when he started taking about attack, slopes and ratio’s having no button’s that say “attack” on my units, just gave up.
keep shinin
jerm
I forgot that most of you are “rockers” and that the thought of elfs, campfires, “smoke”, rollin papers etc., gets ya goin lol
The objective of LearJeff’s Compression lesson was to bring notes that are low in volume, up, more equal to the other louder notes on that track so one could master the song an elfin tad easier.
LearJeff has a good command of the language… There was some pretty clever guys around this board… he sampled a Wurlitzer Electric keyboard I think… OR was it a Fender Rhodes Piano. 
? The samples are posted up here… somewhere…
Anyway, I’d love to have those Wurlitzer Keyboard Samples…
Bill…
I’m not a rocker but I am old enough to be sitting in one for long periods of time.
Sorry, I couldn’t help my elf. I want to thank you falettinme be mice elf again.
Twittle de dee twittle de dumb, Seems that I just cannot quiet down!
Quote:
I'm not a rocker but I am old enough to be sitting in one for long periods of time.
Sorry, I couldn't help my elf. I want to thank you falettinme be mice elf again.
Talk about command of ze English language
Well done 
Elvish?
CaReFuL
TomS…
yOU’RE ALMOST OVER MY EAD…
Stop That… lol…
Bill…
Aw, you guys don’t know nuthin,
http://en.wikipedia.org/wiki…Agin%29
[quote=sevenOfeleven,Mar. 05 2010, 2:31 PM]
Quote:
I’m not a rocker
Quote: (TomS @ Mar. 05 2010, 8:22 PM)
Man...
Do you know how far back they go?
Bill..
Not that far really Bill if you compare them to, Robert Johnson, Bessy Smith, “The Mozarts”.
You know, I didn’t realize it before but it’s now obvouse to me that TommyS is a funk player or just simply funky
Yes, yes, I know how far back that goes, thank you very very very much. YOU SAYIN I’M OLD OR SUMPTIN? Just wait till I get my teeth in, and my orthopedic shoes, and we’ll have a nice little chat over some noncaffinated tea and perhaps a cookie or…uh…what was i saying?
Sad all the history of the N track board has died. No one knows who Limey, Learjeff, Teej, Willy, Pooblio or any of the other 100 old timers are any more.