Compressors… 99% of us use them and most of us have a solidunderstanding of what they do, when and how to use them, and, critically, how to get what we need from them. But I’m going to make the bold assertion that very few of us truly understand how they work and exactly what all those controls do on a technical level – and that includes some of the people who design and manufacture hardware and software compressors.
This lack of understanding isn’t helped by a number of myths about how compressors work and how the controls on the front panel are able to do what they do which are perpetuated by bloggers, forum posters – even books and magazines.
This month I’m going to look at a handful of the most common myths.
- Myth #1: Attack is the time it takes for a compressor to begin compressing once a signal crosses over the threshold
This may be my favorite audio myth of all time, because I think it’s the most pervasive. People who know a lot of things; people who’ve written books on the subject of recording and processing audio, have perpetuated this myth by writing about it, repeating it and passing it along as established fact so that you can read it passed off as fact in countless blogs and forums.
The only problem is that it’s completely, utterly incorrect.
And so, in the pursuit of truth, justice, and all that is good in the world, I give you the correct definition of attack:
Attack is the length of time it takes a compressor to apply roughly two-thirds of the targeted amount of gain reduction.
I say ‘roughly two-thirds’ because there is no agreed-upon, industry-accepted standard for what this spec actually is. Yes, you read that right: no two compressor designers will agree on exactly how to define, and therefore measure, attack. My definition above is within the ballpark of most thinking, so I’m running with it.
To understand this definition of attack better, you need to get some basics of compression established first. Let’s say your compressor is set with a threshold of -10dB and a ratio of 3:1. If you feed this compressor a signal at -11dB, nothing happens because the signal is lower than the -10dB threshold.
But if that signal jumps to -1dB things get interesting. Most notably, the instant the signal reaches -10dB the compressor begins attacking it. There is no delay whatsoever in this response, which belies the myth that attack is the time it takes a compressor to respond once a signal crosses
threshold.
With a -1dB signal and a -10dB threshold, the signal is 9dB over threshold. Our 3:1 ratio means that for every 3dB coming in over threshold, the comp wants to allow 1dB out the backside. Since our example has a signal 9dB over threshold, our hypothetical 3:1 comp wants to compress those incoming 9dB into 3dB at the output, which would require 6dB of gain reduction.
Given that attack is the time it takes a compressor to apply roughly 2/3 of the targeted gain reduction, the attack in this case indicates how fast the comp will apply the first 4dB of the target 6dB of reduction.
If you don’t follow the math of this illustration, don’t worry. For now it’s enough to know that the compressor starts applying gain reduction as soon as the signal crosses the threshold. Which means that attack is not a delay before action, nor is it even a measurement of time per se; instead, it is a rate, a measurement of the speed at which the process of gain reduction is occurring.
- Myth #2: Release is the time it takes a compressor to release
compression after the signal drops below threshold
Without going into detail, let me just say that the above definition is not only incorrect – it would actually be an impossible thing to assign a single value to. (Which is a story for another column.)
The correct definition of release will come as no surprise given what you’ve read above:
Release is the time it takes a compressor to restore two-thirds of the reduced gain to the compressed signal.
‘Restoring reduced gain’ is a very carefully chosen set of words. I characterised release in those terms because it’s useful to think of compression as a two-way street.
When a compressor attacks, it is applying gain reduction – it is lowering the signal level.
But gain reduction is only half the picture, because for every dB of gain a compressor takes away, at some point it has to put it back. And that process – let’s call it ‘gain restoration’ – is the business of release. The faster your release, the faster the compressor restores the gain it took away when attacking.
So what do we know now, at least in a purely academic way?
Attack is the length of time it takes a compressor to apply roughly two-thirds of the targeted gain reduction.
Release is the length of time it takes a compressor to restore roughly two-thirds of that reduced gain.
This gives us a good grounding to tackle more compression myths.
- Myth #3: A compressor won’t release until the signal drops below the
threshold
If you’ve been paying attention, it should already be obvious why this statement is false. The explanation lies in the fact that aside from generating ancillary effects like distortion and colouration from transformers and tubes, attacking and releasing a signal are the only two things a compressor can do.
Put a little differently: any time the gain reduction meter on a compressor is moving, it is either attacking or releasing the signal.
Fascinating! Taking it a step further:
Any time the gain reduction meter is increasing (i.e., the comp is reducing the gain of the signal), the compressor is attacking.
Any time the gain reduction meter is decreasing (i.e., the comp is restoring the gain of the signal), the compressor is releasing.
So while the well-intentioned myth-spreaders out there would have you believe that attack and release are only relevant when a signal crosses the threshold – attack on the way up and release on the way down – what I am telling you is that nothing could be further from the truth.
Instead, once a signal is over the threshold, both attack and release are constantly at play.
There’s a simple way to confirm this. Feed a drum loop into a compressor and set it up so that the signal is always over threshold and the gain reduction meter is dancing between say 6 and 12dB of
reduction. In this instance the compressor is constantly attacking and releasing the signal, as indicated by the dance of the meter.
If the myths were true – if attack only happened when a signal crosses above threshold, and release only happened when a signal drops below threshold – adjusting the attack and release knobs in the above scenario wouldn’t make any difference because the signal is perpetually
over the threshold … but turn the attack and release knobs and you will very clearly hear the sound of the continuous compression changing. Give it a try.
I think most people who use compressors on a regular basis already understand the above on an intuitive level, but some never make the connection that the behaviors they’re hearing (and seeing on the meters) don’t comport with the conventional – and flawed – wisdom.
- Myth #4: Compression reduces dynamic range
How many times have you read this particular nugget of wisdom? And sometimes it’s true. But not always. Indeed sometimes it’s important that it’s not true.
Imagine a mix in which kick, snare, and cymbals/overheads feed a drum bus. The intuitive thinking goes something like this: if I slap a compressor on this bus and compress it, by definition I’m going to be pushing down the loudest stuff and as a result the dynamic range will be reduced. That’s what compression does, right?
Yes, and no.
Yes, a compressor can and does push down on the loudest stuff. But no, that doesn’t mean the dynamic range is automatically reduced, and here’s why: if your attack is slow enough, the loudest bit of that transient will come screaming through before the detector tells the gain circuit ‘TURN IT DOWN! Then, if your threshold is low enough and your ratio is high enough, what does get pushed down gets pushed down so far that the resulting signal is much quieter than it would have been if you hadn’t compressed it at all.
The result of those two factors: the loud stuff is just as loud (albeit for a shorter time) and the quiet stuff is quieter. Which is to say that your dynamic range is now increased as a result of the way you applied the compression.
Engineers exploit this reality every day on their drum buses; the classic trick is to take a comp set to a medium or high ratio, slowest attack, fastest release and dig in hard. With a deft set of
hands and ears, the result is a track that, on its own, is an unusable series of fast, dead-sounding thumps and pops that herald each drum hit in a highly exaggerated but uniformly level manner. This track is then blended in parallel, usually quite subtly, and the result is a palpable increase in the perceived impact, punch, warmth, and consistency of the drum sound.
So yes, compression generally does reduce the dynamic range, but it doesn’t have to, and sometimes it does exactly the opposite to wonderful effect.
- Myth 5: Compression makes sounds bigger
This final myth is very personal to me.
I had the pleasure of attending an early Mix With the Masters seminar hosted by one of the acknowledged masters of mixing and, in particular, artful compression, Michael Brauer.
At one point the group was talking about compression, and someone asked Michael what he’s listening for when dialing in one of his elaborate compression schemes (if you haven’t read up on his multi-bus and five-compressors-as-one-vocal-comp techniques, you should; even if you never try them your brain will appreciate the novel approach).
This is my interpretation of what he said (and I’m OK repeating it here because I’ve since read it in interviews he’s done): pushing a sound into a compressor is like pushing an object into a stretched rubber band. The harder you push the object, the more the rubber band pushes back. Michael listens for the point where there’s a musical push-pull movement and the comp feels springy and flexible.
Not pushing enough results in too little resistance – no interesting movement. But push too far and the rubber band loses its elasticity and becomes stiff – the sound loses its life. What’s more, when you push too hard into a compressor the sound becomes small.
When he said that last bit, I remember jolting upright in my seat because I’d never previously felt like I had a masterful grasp of when to stop laying in with a compressor. I had become pretty adept at using ratio and release to control the transparency or audibility of the effect, and I was starting to feel confident in knowing what kind of attack served the sound in the mix. But where to park that threshold was still a mystery to me and had been for a long time. This nugget of insight felt like the key to solving that puzzle.
When I got back to my room in the States I immediately laid into my compressors and started listening not just for snap and swing but also for size. I became obsessed with running every track I had – every sound and bus, even my FX – through the different comps in my rack and plugin folder.
I relentlessly tweaked them in all kinds of ways – aggressively, musically, invisibly, whatever – constantly level matching and bypassing the comps to listen for one thing and one thing only: how big or small the sound became in the context of the full mix.
What I heard was a revelation. I realised I had been confounding ‘density’ with ‘size’. That seemingly small syntactic error had huge ramifications, both on my productions and on my experience of creating them. This mistake explained why I never knew when to stop digging in with a compressor.
Here’s what that mistake looked like: if I was squeezing a sound and it got thicker, I thought that was the same as making it bigger. I was enamoured with the ‘grr’, the ‘hair’ and the urgency that compression added to my sounds. When I bypassed and that density went away, I was resolute that the compressor was improving things.
Wrong.
The problem with making density your primary compression benchmark is that you can keep going as far as the comp will let you; if urgency is a drug, compressors are the dealers of the stuff. And they have no conscience; they’re happy to dose you up as often and as hard as you’re willing to go.
But mixing is a game of balances. Of relentless trade offs and compromises. Ultimately you don’t want every sound to be as dense as possible; instead, you want it to be as dense as necessary to transmit the emotion… and no denser.
That means attuning your ears to the proportionate spaces around each tone like the curves and twists of the pieces in a jigsaw puzzle, filling up the spectrum where necessary while preserving enough dynamics to allow the sounds, and with them the entirety of your mix, to breathe – to have air around the elements such that you feel the impact when those spaces contract and the sounds collide.
Everything in a mix must be shaped with complete awareness and respect for every other piece in the puzzle… or it won’t fit. It won’t assemble into the vivid picture that the song wants to be – a gripping story the listener wants to surrender to from start to finish.
