Compression Boot Camp

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A great tutorial on using compression by an HRC regular and current staff member.

Without going into what compression is, we're gonna dive right into the nitty-gritty of getting the right settings for the many jobs they can be used for.

There are many different compressors out there, and just like anything else, not all sound/behave the same. Your ears actually have a kind of biomechanical compressor built right into them, as well as tape (saturation), amplifier (over drive), and even speaker (excursion) can compress audio in their own unique ways. So how do we know what's doing what? Simply by listening.

The trick to 'hearing' compressors is understanding how they affect sound. You're not really looking for a 'tonal' type of a sound (well, later you will) but you want to listen for 'when' and 'how much' are they working. Everyday terrestrial TV and radio use a TON of compression darn near all the time (sans HD and other digital type stuff)....flip to a pop or rock radio station and you will little to no volume change (dynamics)...EVERYTHING SEEMS TO BE COMING RIGHT AT YOU ALL THE TIME...ESPECIALLY COMMERCIALS... Now flip the dial down towards your classical or jazz station and notice how quiet it is....what do you do? Turn up the volume a bit (some people think you shouldn't have to do this)...things can get very quiet then suddenly JUMP OUT at you. Flip back to the hard rock station and it's FREAKING LOUD!

The same concept works with individual tracks....and this is how you get everything to 'sit nicely' in the mix.

I prefer headphones at a moderately quiet level....basically turn it up to 'normal' then back off about 15% (try going even quieter). When setting my compressors, for some reason, my monitors just don't let me 'feel' it the way headphones do.

Most of your compressors will have these basic settings:

Threshold - Threshold is the where the compressor will start compressing this is always gauged in Db's. Since a compressor is a dynamics (volume) processor, we will be watching our volume meters quite a bit (both before the compressor and after) . So say your signal goes past -15Db and this is your threshold setting, your compressor will then start compressing (attack) the audio. Then when it goes under the threshold it will stop compressing (release).

Ratio - Ratio will always be shown as "x:1". This is how much do you want to compress the signal. If you have a 2 to 1 ratio (2:1) this means for every two db's of volume your signal goes up, the actual output will only be 1 db louder. This is a relatively modest setting....a more aggressive setting like 15:1 will 'squish' anything that it encounters, the signal would have to go up 15db's before it will yield 1db of volume increase. So in our -15db setting of the threshold, you can go all the way up to 0db before the compressor, and the compressor will only output -14db at the most.

Attack - Attack is a time setting (usually in ms). This is when you want the compressor to 'start compressing' after the signal goes above the threshold. A slower attack time will allow more initial transients to pass through before it starts to kick in....some longer times can be used for entire sections of a song (think RMS levels). Faster attack times will quickly 'grab' the audio and start compressing almost immediately (think PEAK levels)...these quicker settings can be used for, say, a snare hit or an initial plosive of a vocal track....extremely fast settings combined with a high ratio is when a compressor becomes a limiter, essentially putting a limit or ceiling on the maximum amount of headroom.

Release - Release is basically the opposite of attack. This is when you want the compressor to 'stop compressing'. A slower release time will gradually 'let go' of the signal, and a fast release will quickly 'stop compressing' the signal as soon as it goes below the threshold.

Knee - Knee is 'how' the compressor reacts when it starts compressing. Think of it as a person sitting in a chair. Your feet on the floor represents the signal at -infinity db (no sound). As the signal travels up the leg (we're talking volume here) it gets louder and louder. Then it hits the knee, this is where the threshold is crossed. If you're sitting with your legs going strait down (90 degrees), that's known as a hard knee, as the compressor essentially works quickly and harshly. This is good for limiting purposes. But say you don't want it to be so rigid, then a softer knee will be like standing up a bit to where your knees make an obtuse angle, letting the signal slowly get louder as it further exceeds the threshold.

There are other controls on some compressors, but these are the most important, without getting to the input and output gain which is more of a gain staging thing.

Okay, so lets say you have a bass track that needs some taming...

Start by setting everything to zero....ratio is 1:1, attack and release times are as fast as possible, threshold is 0db (or all the way 'up').

The compressor should not be doing anything to the signal. Turn the ratio to something high like 15:1 or higher, again, this will not affect the signal yet because we haven't set the threshold yet. Now lower the threshold to where you can hear it start AND stop compressing on a regular basis (kind of a 'pumping' sound. You will hear the volume kinda 'suck in' when it's compressing and then get louder on the quieter parts....this is when you can set the attack and release times fairly easily. Because you can drastically hear when it's kicking in, you will be able to clearly hear when it starts and stops compressing.

Slowly adjust the attack time to where it sounds 'natural' no mind to the fact you're squashing the signal still, you wanna listen to 'time' right now, not 'sound'. Do the same with the release, go all the way up and all the way down, and aim for the sweet spot when you want the signal to 'breathe' naturally with the song. Once you get the attack and release set, ease up on the ratio some until you get a nicely 'controlled' sound when the compressor starts working. The object here is you don't want to hear it working. Now adjust the threshold to a point where the compressor is working (watch the reduction meter if you have one) but not working all the time or too hard. It should breathe and play nicely with the track. Again you want a 'natural' but controlled sound. You may want to adjust the threshold and ratio a bit to find the sweet spot.

With a bit of practice on different sources like percussion, vocals, and even entire songs, you can get the hang of compressing with confidence instead of relying on presets and hoping for the best.

happy squishing,

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User-submitted comments

Dec 09, 2007 05:07 am
I forgot something!!!
[b]Makeup Gain[/b] or [b]Output Volume[/b]

this is a control because when you compress, you are effectively reducing the now you can bring the overall volume up after compressing...push this up until you clip then back off a hair.

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