Choosing the right compressor settings
Now your ears are focused on what the settings are doing, where should you set them ? Easy:
Adjust the controls until it sounds right
OK, that's next to useless, I know. "What sounds right" varies on the instrument or mix, the genre and taste. There is also a big difference between compressing an instrument in a mix and compressing a whole mix.
Compressing an instrument
For example, compressors are commonly used to even out very dynamic (spiky) intruments in a mix, like a snare drum, say. A snare note has a big spike at the beginning (the transient) created when the stick bashes the skin, followed by a longer sustained sound as the body of the drum resonates. If you push this uncompressed signal too high in the mix, the initial transient may start to sound annoyingly loud and percussive or even distort, if you push it into the red. As a result you can't hear enough of the "thump" of the drum to give it weight and impact.
To minimise this effect, you can compress the signal, using:
- a fast attack time - as soon as the signal starts to get loud, the compressor quickly starts to take effect
- a high ratio to have quite a dramatic effect reducing the spiky transient
- a fast release time to allow allow the compressor to "relax" quickly once the spike has been controlled
- a fairly high threshold so the ring of the drum is not compressed as well as the transient
Compressing a mix
Using settings like the ones above almost certainly won't work well on a mix, though. As a rule in mastering, the aim is for the compression to be as unobtrusive as possible. Usually we aim for a natural-sounding result, where the compression isn't easily noticed. Most final mixes have already had suitable compression used on the individual instruments, so the settings described above will probably result in something that sounds squashed and lifeless, or even pumping and distorted.
Remember in what follows that it's very common to use both a compressor and a limiter when mastering - the limiter catches the very fast transients, allowing the level to be lifted even further, but I'll discuss limiters in the next post. As far as compression goes, for a whole mix I often:
- Avoid very short attack times - and very long ones
- Use shorter release times
- Use low ratios
- Avoid large amounts of gain reduction
The medium attack time allows percussive elements of the mix to punch through, but still controls the overall signal level. If this gets too long, you'll hear the "thump-suck" effect I mentioned in the last post. Shorter release times also avoid obvious pumping, but you need to be careful. If they get too short, there won't be enough control and you may hear the compressor "bouncing" - rapidly triggering and releasing over and over, sounding crunchy or distorted as a result. Low ratios and moderate gain reduction avoid the compression becoming too ear-catching. Time for another rule of thumb:
If you compare the compressed signal with the level-matched original, it should sound better
Meaning not squashed or pumping (unless that's what you want !). You should always level-match the "before" and "after" levels before making your comparison, of course. When level-matching for comparison purposes, it's often worth using the vocals to judge the loudness. Our ears tend to latch onto a voice as the most important element in a mix, so get these as close as possible before comparing. Then listen and ask yourself questions like:
- Which do I prefer ?
- Am I sure one of them isn't louder ?
- Have I achieved what I wanted ? (ie. made it punchier, fuller, with more impact & excitement)
- Does the compressed version still sound lively and exciting, or is it too squashed ?
- Does it sound closer to similar tracks I'm trying to emulate ?
- Can I hear more of the quiet details in the mix, or is it getting "mushy" and confused ?
- Does it still sound natural ?
If you can answer these questions with a positive, you're doing well. If not, try varying some the settings and comparing again. If you still can't get a result you like, try a different compressor - or maybe it doesn't need compression at all.
All of this is very hard to describe in words of course, but hopefully by now you'll have a better idea of how compressors are used, what the controls actually do and what you should be listening for.
If you found this post useful you might also like to watch my free webinar on the more advanced technique of multi-band compression - for more information, click here.