I had always thought that the attack and release values on a compressor denote the actual time it takes for the gain reduction (determined by the overshoot amount and ratio) to take place in full. But in the chapter on compression in Roey Izhaki's "Mixing Audio," he says:

"It is important to understand that both times determine how quickly the gain reduction can change, and not the time it takes it to change. In practice, both define how long it takes the gain reduction to change by a set amount of dB. For example, 1 second of release time might denote that it takes the gain reduction 1 second to drop by 10 dB."

If this is the case, then wouldn't trying to compute attack/release settings according to note values and tempo (which I've heard of people doing) ultimately pointless?

It gets even more complicated when you muddy the waters with source-dependent release times (e.g. LA-2A), automatic release time adjustment, and non-logarithmic release curves (e.g. 1176). I like a calculator and a graphical display as much as anybody, but setting compressor release times is one area where you have no choice but to listen closely and - as much as I hate the phrase - trust your ears.
 
Digital compressors are quite a bit more complex than their analog forebears, where the release control was nothing more than a potentiometer in a simple R/C circuit - essentially the same as the Release portion of a synthesizer's ADSR. Simple and predictable. Nowadays, compressors can have fully-customizable transfer, threshold, attack and release curves.

Or put another way, it gets less complicated when using a compressor with auto release, at least on the user's end anyway. There's certainly no calculating release times via bpm. Waves H-Comp has the option to sync release times to the project tempo but if release time is a measure of how long it takes for gain reduction to reach a certain level then I'm not sure what the point is because it's not as if gain reduction is guaranteed to reach 0dB in the chosen note value.

I always thought release was the easiest compression setting to hear anyway, especially when it's master bus compression. There's usually one release setting that just sounds way better than all the others.

The thing about Release settings is that the time per quarter note is sort of relevant but only if there is a peak that is triggering the threshold every quarter note!  There may not be so then that time maybe is of not much use.
 
Using your ears is certainly a better approach. 
 
As far as Attack is concerned that to me is an ears only thing. There is no real way to determine by calculation what the attack time should be.  The best way to determine attack settings is listen to the leading edge of the sounds or attack transients.  Note how with very fast attack settings the transient gets destroyed.  As you slow it down, hear the transients and the punch come back.  When you are wanting your compressor to act as a limiter though this does not apply so much because the peaks will still get through.  Set the Attack real fast but the Threshold high so it only acts on the loudests bits that you are wanting to limit.
 
Dont forget there is a good order to adjust these things.
 
A - Attack
R - Release
R - Ratio
T - Threshold
 
Any other order and you are chasing your tail.  I believe anyway the order is more important than anything.  Whenever I do it this way I get the nicest and most transparent compressor sound.
 
I did write a thread on this at some stage:
 
http://forum.cakewalk.com...to+set+up+a+compressor
 
I have been getting into Multiband compressors much more lately (mainly for mastering over a whole mix)  Yes they are harder to set up for sure but very worthwhile usually.  The Cakewalk Multiband is a ripper as well.  Great GUI too.  Easier to setup than many.
 
 

Strict calculations do not work for everything, and compression is one of them. This is why presets for compressors simply don't work other than the odd happy accident. Every track is different and needs to be treated differently.
 
The only preconceived thought I have when I put a compressor on something is that I like certain ratio's on certain parts and even that is subject to change. Attack, Release, Threshold, Input (if available), etc...  are a blank canvass for me every time I add a compressor.
 

Jeff Evans
Dont forget there is a good order to adjust these things.
A - Attack
R - Release
R - Ratio
T - Threshold
Any other order and you are chasing your tail.  I believe anyway the order is more important than anything.  Whenever I do it this way I get the nicest and most transparent compressor sound.

 
This is interesting Jeff.... I find the controls are so interactive that adjusting one may require a change (often very small) in another - I guess this is what you call "chasing your tail". So my question becomes.... if the compressor is not compressing anything (ie: you have not adjusted your threshold or ratio yet) how can you hear/feel the attack and release to set them? 

I would say that you lower the threshold low enough so that you can hear what the attack is doing, then set the ratio and release, then raise the threshold again until the desired amount of compression is happening.

I thought I had explained it in my compressor thread but will elaborate here.  You do need some temporary settings for sure as you adjust each parameter.  Sharke is a little incorrect also. You set attack first, then Release then Ratio and Threshold last. (Important!)  The good thing about this approach is that adjusting them in this order prevents you from returning back to a previous setting.
 
To set Attack start with a high ratio eg 10:1.  Set the Release time for a very fast Release.  Lower the Threshold now so the compressor is working.  It may be pumping a little but ignore how it sounds and only focus on the sound of the Attack transient.  Listen to the thickness of the Attack transient.  Too fast an Attack creates a thin sounding transient.
 
Even with the same settings you can set the Release now so the sound bounces back at you or glides back towards you.  A bit like a swing.  The Release time should start to be longer.  Make the rush of the Release a musical component of the sound.  It should push you into the next beat without pre empting the beat.  Let the compressor push the sound towards you until the music makes its next statement.
 
The extreme settings will all help to emphasise the Attack and Release settings.
 
At this point the Ratio is still high so things will sound over compressed.  Lower the Ratio now as much as you can without the losing the effects of the Attack and Release settings.  Think of the Ratio setting as the size and firmness of the sound.
 
Last is Threshold.  Adjust it so it the compressor is not compressing all the time.  The right setting will see the dynamic movement coming to rest at special moments, otherwise you get a flatter more lifeless sound.
 
Hope this helps.  A lot of this is also in Mike Stavrou's great book 'Mixing With Your Mind'
 
 
 

When I said lower the threshold first, I meant so that the compressor is actually working and you can hear the attack, not actually set it first. I believe that's what you're saying also? Still set the threshold last, but at least lower it first so that the gain reduction kicks in.

Yes good point. (Sorry, my confusion in how you worded that) The main thing is to set the ratio and threshold using some temporary settings just to get the compressor really kicking in and working even slightly hard. Then you can focus and Attack and Release.
 
This is not a hard and fast rule either. Setting compressors any other way will result in a different sound and that sound may be valid too.
 
Mike describes it well by saying the compressor is a bit like a safe and tumblers. By setting Attack first you are dropping the first tumbler in place and Release is like the second etc.. By the time you get to the fourth tumbler you can open the door. I believe if you start having to go back and reset things that you set before you could be doing it the wrong way.
 
I used to have an old Yamaha compressor (GC2020) that I could never get a decent sound out of until I started using this approach and then all of sudden it sounded really great.
 
This all applies to virtual compressors too. I even go through this approach for each band of a multiband compressor as well.  It takes longer but you end up with a sweet sounding and seriously transparent sounding compression.
 

Jeff Evans
I thought I had explained it in my compressor thread but will elaborate here.  You do need some temporary settings for sure as you adjust each parameter. 

Sorry Jeff, I didn't mean to make you repeat it. I have no excuse for not clicking on your link before asking about it. Thanks for the detailed explanation. I'm not sure this is much different from how i actually do it, but I have taken note and will keep this approach in mind.