Typically in mastering, we are dealing with varying levels of passages as opposed to something like the loose dynamic range of a bass guitar in mixing. In most cases, attack times should not be too short as not to risk damaging the important transient hits. They may start at around 20ms going up to as far as 100ms. Release times may be somewhere between 0.2s (200ms) to 2s (2000ms) sometimes as long as 5s. There is no real answer, it is completely dependent on the music at hand. You may, or may not be feeling confident at this point when determining your attack and release settings - it takes practice. The instructions on this page will give you an idea of where to start.
The following technique assumes the track being mastered is of a typical commercial nature and has a kick and snare providing the rhythm.
At this point I shall tell you that your compressor will most likely respond more to the kick than any other elements (providing the kick has been mixed at a proper level), you will soon discover why.
Can you distinguish the natural rhythm of the track? Can you nod your head back and forth with the rhythm? It's important that you can as this will help you decide upon attack and release times.
First choose a suitable ratio, no more than around 3:1. Relatively low ratios are best in mastering. Too much will result in a pumping effect of the compressor, unless that's what you're looking for. Some forms of dance music actually benefit from a little rhythmical pumping from the mastering compressor. For the technique I am going to demonstrate, the compressor will be set up in a relatively transparent way (undetectable to the listener).
The quality of the compressor being used may dictate how extreme you can go with your ratio.
To start with, set the attack time to be a medium amount, say around 30 to 40ms as we know that too short a attack may damage the transient hits and take away the punch of the music. The release time will be quite dependent on the rhythm of the track, for now just choose about 500ms. Select a medium to soft knee setting. A soft knee tends to give a more pleasant result in mastering as it sets the compressor to react in a less aggressive way to the threshold being triggered. Hit play and start to pull the threshold down - bring the compressor into an almost constant state of attenuation. You will notice a significant drop in volume. Use the makeup gain to bring the volume back up to a working level. Now it's time to use your ears to set the attack and release times.
If you shorten the attack too far, you will hear a reduction in the track's punch. Turn the attack too long and too much audio will have passed through before it has time to react. You will hear the compressor lagging behind, as if it can't keep up. It will really take away the transparentness of the compressor. There's a sweet spot for every track - with practice you'll feel it. Bear in mind that it may be after you've experimented with the release before you find the perfect attack.
The release time will really set the compressor swaying. Too short and you will hear the compressor aggressively jump back at you after every transient impact. Too long and you will feel the lagging behind effect again, along with a huge amount of attenuation. The real sweet spot is where the release begins to compliment the attack. Try to imagine it swaying back and forth to the rhythm. Again, there will be a certain amount of tweaking both attack and release together to find that sweet spot – try not to look at your compressor's metres, get used to hearing the effects with your ears.Here's a way to visualise the attack and release in a mastering situation. Imagine just a kick and hi-hat. The kick happens every beat, the hi-hat on the 8th's - all at about 100bpm. The kick's level is above the threshold and so it will trigger the compressor but the hi-hat's level is below. The kick triggers the compressor and can be noticed in the sudden reduction in the level of the hi-hats. As the compressor recovers, you would hear the hi-hat's level gradually rise back up. In such a scenario, if the hi-hats where to arrive back at their full level before the next kick triggers the threshold (the release time being shorter than the time between each kick), then the result would be an awkward jump back to the full level. The awkwardness coming from there being no real significance at the point between each kick where the release time stops and the compressor stops compressing (unless you get it bang on half a beat or something). If the release was set to be only just longer than a beat, the compressor will still be in a state of compression when the next kick triggers the threshold. This takes away the awkwardness of the compressor jumping back and resting at no significant time, the result being smoother and more flattering to the music.Obviously, the likelihood is that the music in question is probably much more complex than just a kick and hi-hat, and there will be a whole array of different sounds triggering the compressor at various times, but this concept can still be applied. Having the release that tiny bit behind the natural rhythm can give you a smoother, more flattering result.
Put the makeup gain back down to 0dB and go back to the threshold. At this point, there's a chance that we might be squashing the track a little too much for mastering purposes. Typical levels of gain reduction should not be more than about 3dB in most cases. More than that and the transparentness of the compressor may be taken away. Perhaps take the threshold back up a little to find a suitable amount of attenuation, then reset the makeup gain to compensate for the loss of level. Be aware that further adjustment of the threshold can affect your other settings - certain elements in the music may cease to trigger the compressor having raised the threshold.
The real transparency of this technique comes from the compressor moving in a natural way to the rhythm and so blends in with the musicality of the track. It's when your settings are too harsh, aggressive, too fast or too slow that the compressor falls out of sync and exposes itself as being an artificial element in the music.
On the next page is a video of this technique using Logic's own compressor.
I'm afraid there is no compressor setting that fits all. The attack and release times will vary massively from track to track. There's no real relationship between the BPM of the track and setting the attack and release times. It's all about the feel of the rhythm.
David Eley - TGM Audio
Next page... Demo of Compressor Technique