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Compression and limiting - the Punchbag Analogy

February 16th, 2011 BY 

How do you know the best settings for a compressor or limiter ?

That's a question I get asked all the time, especially in attended mastering sessions. And when I'm answering it, I almost always end up punching my fist into my hand.

This post will tell you why.

First of all though, very quickly:

What's the difference between a compressor and a limiter ?

The answer is that a limiter is just a compressor with a very high ratio. Typically they have very fast attack and release times, too. (If you're not quite clear on exactly what a compressor is or how best to use one, read my post on Using Compression first.)

So, limiting is a pretty aggressive form of compression. This means that if you push something too hard into a limiter, it will very quickly have a negative effect on the audio - you'll hear it go blunt and lifeless, with a loss of excitement and impact, or even distort. On the other hand, used lightly on peaky material, it can be a very transparent way of increasing the level.

How do you use them ?

At this point people always start asking me for rules of thumb - favourite settings, what instruments you should use compression on, maximum gain reductions and so on.

But I never have any easy answers ! It always depends on the material, the sound you're trying to achieve, and the particular box or plugin you're using. And the natural next question is - how do I know what it should sound like ?

And there's no easy answer to that, either. It's a combination of instinct, trial and error. I find these are tough things to get right too, even though I've been doing it for years.

But I know how it feels to me, when I'm making these decisions, and that's why I start punching my fist into my hand when I try to describe it.

The Punchbag Analogy

Imagine you're a boxer.

(I've never boxed in my life by the way, so if you are a boxer, and what I'm saying here is complete nonsense, you'll have to forgive me. I still think it's a great analogy : )

Imagine you're practising your swing, with a great, solid, punchbag.

Every time you connect there'a satisfying "thwack" and you can feel your glove driving into the bag. It gives way slightly, absorbing the impact, and swings back a little. There's a really nice balance between the power of the punch, the force of the impact and the response of the punchbag. You can build up rhythm and momentum, and every punch is clear, hard and clean.

That's how it should feel when your compression and limiting are really working right, and that's my Punchbag Analogy.

Still confused ? OK, let's break it down a little.

Why a compressor is like a cushion

Let's talk about cushions. No, really !

Imagine for a minute you're punching a cushion. The positioning of the cushion is like the threshold of the compressor. If the cushion is too far away, you're just swinging your arms around in thin air. That's like having the threshold too high - the audio never "makes contact" with the signal, and the compressor doesn't do anything. If the threshold is too low, the cushion is too close, and you're always touching it - you can't get in a decent swing.

The softness of the cushion is like the ratio. Imagine punching a pillow - that's a compressor with a very low ratio. Even when you cross the threshold (hit the pillow) it's really soft and barely slows your fist down at all. Now think of punching a bean-bag instead - that's a higher ratio, it slows your fist down quicker and offers more resistance. It's also deeper and heavier than a pillow, though, and that fits the analogy, too - the size and depth of the cushion determines the overall gain reduction.

Now think of punching a sofa cushion - the firm kind you sit on. It's made of foam, it's pretty effective. In my mind, a sofa-cushion has a ratio of about 2:1, whereas a good, solid leather cushion probably has a ratio of 4:1  ;-p

Finally, imagine punching a crash-matt, the kind they have on the floor in a gym. Up against a wall, a couple of inches think, and actually pretty hard. That's a 10:1 ratio or higher, and the gain reduction is pretty extreme. It's a blunt, jolting impact - not something you want to do for long.

Back to the punchbag

Of course no real boxer is going to practise punching pillows, cushions, or crash-matts ! A real boxer has a high-quality punchbag, carefully chosen for his or her body-weight and strength.

First of all, the punchbag is the perfect height and distance for the boxer - just as, in a compressor, the threshold needs be set right for the incoming signal level.

Next, the punchbag is just the right balance of soft and hard - in fact, it's probably slightly softer at the edges, getting firmer inside: that's a "soft knee" compressor, where the ratio increases gradually just above the threshold : )

Its also the perfect weight - enough to make the punch feel satisfying and powerful, but not jolting or jarring. That's the balance in a compressor between the ratio and the gain reduction - too high a ratio and too much gain reduction, and it's like punching a very thin crash-matt right up against a wall. It hurts.

We can even imagine how attack and release times come into this - a springy, elastic punchbag might quickly regain it's shape (fast release) and allow the punch to "bounce", whereas a denser material might take longer and absorb more energy from the punch.

(And, by the way - I always imagine a brick-wall limiter to be exactly what it says - like punching a brick wall. NOT something you want to do without gloves : )

Taking the idea of "punch" to a whole new level

So how is all of this supposed to help ?

Just try imagining your audio as a punch. If it was a punch, how would you want it to feel ? Hard and jolting, or soft and consistent  ? Do you want the instrument really "in your face" ("up against the wall"), or just in a nice, rhythmic groove, with a satisfying weight ? Or maybe it's not a punch after all, and a compressor isn't the right tool to use ?

Maybe none of this makes sense, and it's just a personal thing for me !

One thing is for certain though, whether this post has made contact with you or not - using only the "crash-matt against the wall" or even "bare-knuckles-on-brick" approach on everything is never going to feel good for long… sooner or later you just end up feeling "Broken, Beat & Scarred".

Oh - and, why do I always end up punching my own hand ? Because it's a great way to illustrate how you want something to feel, and how you should use a compressor to achieve it. Try it ! Imagine an instrument or mix, and punch your own hand to show how it should feel when you hear it. Then imagine the ideal cushion you'd need to achieve that feel... and set your compressor to match.


(To see a video showing how to use compression to master a song really loud, without losing too much punch - and, the price you pay for it - click here.)

Does this analogy make sense to you ? How do you feel or think about compression when you're using it ?

Original image by Profound Whatever

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My name is Ian Shepherd - I've worked as a professional mastering engineer for over 20 years and I run the Production Advice website with over 50,000 readers each month



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