Mastering Techniques - Using A Compressor - Part 2 - Mastering Media Blog

Thursday, 21 August 2008

Mastering Techniques - Using A Compressor - Part 2

This is Part 2 of a series of posts about using compression in mastering - the first post is here. For a more general introduction to using compression, click here.

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
With a little tweaking, the result should be a much fuller, punchier snare sound - the compressor jumps in quickly (fast attack) and holds back some of the big spiky transient (high ratio high threshold) but because the spike is very short, quickly releases again to allow the main note of the drum to sound. Overdoing it will have undesirable results though - for example if the attack time is too fast, it will remove all the attack from the note, making it sound dull and lifeless. But a bit of experimentation will soon sort this out, and as a result, the snare can be lifted higher in the mix without becoming annoying or distorting - the compression makes it louder. It also sound fuller and punchier, as a by-product.

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.



ca said...

This is an excellent article. Always good to read about techniques from real mastering engineers!

One thing i've found works really well is chaining two compressors, the first in parallel with 2.1+ ratios and -6db gain reduction and then a second stage with a high ratio, -5db threshold but with short attack a and moderate release. This way you bring the program level up without damaging transients, but still get loudness, but also clarity and a bizarre but intimate "sticky effect", perfect for electronic music.

I've started a mixing blog that tries to explain in detail, some of the techniques for mixing and mastering. see if you are interested and maybe we can exchange links?

Peter said...

I'm not able to find the multiband compression and limiting post, where is it? Thanks!

ianshepherd said...

Hi Peter,

That's because it's not posted yet... unfortunately I never finished writing it !

The good news though is that there has been so much interest in the DIY mastering posts that I am writing an eBook on the subject. This will collect all the existing posts in one place and also update and expand on them, plus adding completely new sections on dither, writing masters and multiband compression & limiting.

This is quite a lot of work so it won't be free, but I'll offer Mastering Media readers the chance to get it a a lower price at first - maybe it's something you might be interested in ?


Jim said...


I've read much about compressors since I bought mine in the recent past. Your article was by far, the most cogent ... so thanks! I am interested in using my compressor for podcasting - what are the settings you would recommend for the SPOKEN word?


Jim said...

It's Jim again. Please respond to this post, as when you do - I will have notice of it! Thanks.

Ian Shepherd said...

Hi Jim,

Sorry, I missed this comment when you originally left it for some reason.

For voice I would start with fast attack and release times, and a ratio of about 3:1, then use the guidelines in this post to tweak them. The threshold would depend on the input level - I like the compressor to only work on the loudest moments, ie. set it so the "normal" level is just below the threshold and the compressor starts to work on anything louder.

Hope that helps ?


PB said...

So many people give tutorials that say fast or slow and kind of steer clear of actual settings suggestions.
How about throwing some basic numbers out there in addition to fast slow. What's fast, what's slow?
There must be a set of actual settings to start from that I can copy to my dbx 160sl as a template so I can go from there.
Great info by the way.

PB said...

My apologies for my last post. That's what I get for reading part 2 before part one where you did suggest some settings.

woocatt said...

hi Ian!....a quick line to say how glad i am to have found this site's only a few days now but i have learned a lot and i understand things more.. i play in a band and record on a Zoom multitrack and get good results.but i cant wait until sundays rehearsal to record it and use some of the techniques with more confidence and understanding of what i am actually doing. your tips have made me realise alot of the stuff i was doing was so wrong.i look forward to all of your newsletters ..thank you!

Ian Shepherd said...

Glad you found it helpful !

The Telenator said...

Your series dealing with mastering -- compression and related topics -- is long overdue. Of all things musical, I find that this area is perhaps the one that has the greatest number of conflicting opinions, outright incorrect advice and information from young and inexperienced sound engineers, and just plain badly written articles and posts. Thanks for clearing this up for so many readers!

John the Doe said...

Hay Ian
Firstly, thanks so much for taking the time to write such a great, informative blog on compression.

Its defo considered a dark art due it its air of mystery and understanding lol

As others have already said, there is so much conflicting information out there and i have found most of this is due to the simplest thing - its purpose or intended use!

Some people wish to use it for single channels / instruments while many like myself like to now more about how to use the compressor for mastering which, as you have pointed out, are 2 totally different things :)

Im so glad you pointed that out as disinformation on the net is like wildfire.

Just a side note:
When it comes to checking final masters, obviously it should be played on many different stereo systems as well as different speakers..
This being said, im finding major issues with the iphone speaker, which is mono.
Yes i know its a crap speaker but even so, people still use it lol

Ive found that despite taking the compressor and the limiter out of the master the sound still creates a strange compression gain type effect when, for example, the drums cut out, the rest gets louder, and its rather noticeable.

Has anyone else come across this problem or is it just me : /

Thanks again :)

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