New Media Guides

Compressor and Limiters are often part of your audio recording and/or editing software package. 

A Compressor is a programmable amplifier designed to decrease the difference in level between quiet and loud signals. 

A Limiter is a gain (increase in signal strength or intensity) controller. The output goes so high and no further. Anything above that selectable point is cut off and you will not hear it in your final mix.

These two functions are related can often be used together. Think of it this way. If dynamic range were a room, a compressor adjusts both the floor (really loud sounds) and the ceiling (really soft sounds), and a limiter adjusts only the ceiling.

Please listen to my audio explanation for compression. One word of warning... this audio was produced as part of my "Ride To WorkCast" series. That means I have automobile noise as part of the audio since I create alot of material on my 50 minute ride to and from work everyday! 

Now for Compression Settings - Here is a good example of how to set your compression: Taken from tweakheadz article on "How to Record"

Compressor Settings

Setting Gates:  Compressors do add noise to a signal, and they do destroy dynamic range.  Noise is taken care of by gating the signal.  When it dips below a certain threshold, the audio signal is muted.  This is effective for getting rid of low level noise you do not want in the file, such as bleed from headphones, or the vocalist moving, turning pages on lyric sheets, etc.  Gates have two parameters: 1) The noise floor threshold, and the Rate.  The Noise floor threshold eliminates all of the signal when it dips below the threshold, which is set from -50db to -10db. I keep mine set to -30db. Yet one has to be careful.  If the gate is set too high, then the attack of the vocalists words may be cut off or come in too abruptly.  The Rate parameter  "fades out" the audio signal as the gate come on.  This is effective to prevent the gate from chopping off the tails of the words.  Usually a rate of 1-1.5 sec is enough.

Setting Threshold:  The Threshold is the all important level at which the compressor kicks in.  If you set the threshold to -10, it will leave all of the signal under -10 alone.  When the signal exceeds -10 then it starts compressing at the ratio. -10 is an excellent place to start.  Don't confuse this with the fact that your gear is outputting -10 or +4 impedance wise.  Though the threshold seems like it is a volume control, it is not. It is merely telling the compressor at what level compression takes over the signal.

Setting the Ratio   2:1 is probably the most common setting for a compressor recording or playing back nearly anything.  A great starting point. What this means, simply, is that it takes 2 decibels of sound energy to raise the output meter by 1db. You can read the 1st number as the db IN and the second as the db OUT.  Again, 2db IN equals 1 db OUT.  With 2:1 you simply divide by two. 

Answer this: If your vocalist was singing at -10db and suddenly got 20 db louder, without compression, where would the meters post?

  -10+20=+10.  The meters would post at +10

Correct!   Which, as you should know is way to loud and would ruin the track. Now, if you had 2:1 compression applied, where the output is half of the input, where would the output meters post?

 -10+(20/2)= zero db!

The vocalists 20db burst was compressed to an actual 10 db difference in gain. (the ratio 2:1 is the same as 20:10, or half).    (Note, you don't have to record all the way up to 0db, leave a cushion for the best sonics)

 If you had the compressor set at a 10:1 ratio what would that mean?  It would mean for every 10 decibels of gain the meters would only go up one db.  So in our example, then, the 20 db burst would only let the meters go up by 2db  (10:1 is the same as 20:2, or 1/10th of the original sound),  Since they started at -10, the overall level would be only at -8 during the sudden 20db boost.  Hardly any change in the output level at all.  Would that sound "squashed"?  You bet.

Setting Attack and Release:  These settings can be tricky as they can "delay" the effect of compression on the attack and make is hold on a bit too long on release if set improperly.  I suggest till you get these tricky settings figured out (which takes quite a bit of experimentation) you simple use the fastest attack and enough of a release so the vocal is not boosted as the word trails off.  Otherwise a word may pump on you unnaturally.

Setting the output:  This is the final adjustment as the signal leaves the compressor.  It's sometimes called the "make-up gain". They call it that because compression often lowers the overall signal and you may need to boost it back up.   Basically you want to optimize this so it does not ever go over 0db in the recorder.  With luck you should see a consistent healthy level on the recorder's input meters regardless of how loud the vocalist is singing. 

Just a final note, you can compress again after the vocal is recorded as you prepare your tracks for the mix.  So, don't get too wild with settings at the input (recording) stage.  You want the recorded vocal to sound natural, where the compressor just makes it an overall more useful signal to tweak later with exciters, harmonizers, pitch intonation correctors, and effects like reverb, delay. etc. 

Reference: Tweakheadz Lab  "How to Record Vocals" article