Confused by compression? Our resident studio hound Chris Lyth is here to help...
All alchemists and magicians were charlatans. Their trade relied on creating a convincing illusion using tricks and tools. The same could be said of music producers, because recorded music is also, essentially, an illusion. It attempts to put a band in your living room and the listener in the middle of an orchestra, again by using tricks and tools. And few tools have novice producers scratching their heads like compression.
Compression can be tricky to get your head around - and it's not helped by dry explanations that sound as if they've come from a washing machine manual. So let's raise the curtain on this particular tool and learn how to create some illusions of your own.
What is compression?
Compression is essentially a posh automatic fader that can act very quickly if required. It automatically turns down the volume of the louder parts of your sound, which helps a mix by keeping lots of tracks with lots of volume changes in them automatically under control.
Consider, for example, a vocal take that goes from being very quiet to very loud. Compression helps such a vocal sound more consistent and allows you to turn up the overall volume without it being overwhelming during those louder, 'belt it out' sections. With the louder peaks now under control, the more whispered parts will be more audible, as we can raise the overall level without fear of blowing the listener's head off. Aside from altering the dynamic range of a track, compression can add tone and colour, making things sound warmer, thicker or more aggressive, depending on how it's used.
Depending on what compression tool you're using, the controls/parameters may vary slightly, but these are the basics that you can expect to see:
Determines the point where the compression begins to work. If the threshold is set to -15dB, then any audio that is -15dB or louder is affected. The more you bring down the slider or knob, the more compression is added.
The 'ratio' describes the amount of compression. The higher the ratio, the more robustly the compressor affects the signal. A ratio of 2:1 for example will fairly gently nudge the signal back a little, whereas a ratio of 12:1 would batter it down like the Turkish police dealing with an unruly crowd on match day.
Attack & Release
These controls determine how quickly the compressor's gain reduction reacts to changes. Attack specifies how fast the compressor reacts to reduce gain, while release specifies how quickly the gain resets. Every compressor behaves differently so it's not worth getting too hung up on exact numbers - what's important is getting to know how yours behaves.
As stated above, not every compressor design is the same. Some, such as the classic Urei 1176 and its many clones, don't have a threshold setting, having an 'input' setting instead. Others, such as the Waves dbx 160, have fixed attack and release settings. But generally speaking, the controls described above are the ones you'll most often see.
Compression in use
Compression can be applied in many different ways, and for different reasons, but here are some of the most common situations where it might be used:
On a single track
If you have, for instance, a slap bass or another instrument that varies wildly in level, then this would be a good time to break out the compression tools and tighten that sucker up. The faster and denser the notes are, the faster the release time will need to be to allow the compressor to recover quickly for the next note.
Use the attack to change the tone of the compression. To tame hard transients, for instance, opt for a fairly quick attack. For a smoother sound, such a vocal, opt for a slower attack. How much gain reduction you should use depends on how wild the changes in volume are. The wilder the changes, the more compression you will require.
Drum bus compression
The lack of analog tape in modern recording has been a blow to drum sounds in general, but what is it we are missing exactly? Simply put, tape compression and saturation. Hitting the tape hard used to cause transients to get chopped off, making the drums sound fuller and (with a nod to Zammo from Grange Hill) that bit smackier. But by using bus compression we can get a similar effect. What we are going to do is send all of your drums to a stereo submix channel and add compression. This stereo channel is then sent to your main L&R mix.
Use lower ratios between 2:1 and 4:1 to reduce the gain by 2-4dB. Lower ratios will help to thicken and beef up your drums without making them sound overly processed. For more dramatic, over-compressed sounds, higher ratios and more gain reduction are the way to go. As every track is different, so is every setting, so play around to find the perfect fit. The attack and release settings will require some attention. Start with a fast attack and fast-ish release to give you that smack to the drums, or go for slower attack and medium release for a more natural, punchy sound.
As basslines command so much power in a dancefloor track, having one that is dynamically uncontrolled could be a disaster. Basslines often move up and down the scale and some notes play louder than others, so getting them to sit in the mix can be difficult. Some compressors seem to work nicely on bass, and others less so. The dbx 160- or 1176-style plugins would be my favourites.
As a general guideline setting...
Ratio: Somewhere between 3:1 and 6:1
The amount of gain reduction is very much down to a matter of taste and the part being played, but personally I'd set this at no more than -8dB. If you're doing a house track with a real bass guitar, 90 per cent or more of a good bass sound is in the playing. If it doesn't sound close with a straight DI and a little gentle compression, then play it again!
This involves taking a signal (such as your drum mix), making a copy of that signal (either by bouncing audio, or by routing the signal to a separate stereo channel in your DAW) and then adding extreme compression to that copy. Parallel compression works particularly well on drums, making them sound big and powerful, but still retaining the dynamics of the original. It's a real 'best of both worlds' situation.
With many compressors now featuring a 'wet and dry' function, adding parallel compression is fairly easy. Simply adjust the balance of the wet and dry button to hear it in action. A reasonable starting place would be to set the compressor ratio to 8:1-10:1 and adjust the threshold to show -15db gain reduction. Really smash the dog snot out of it! A fast attack of -10ms coupled with speedy release will rev up the pumping effect, but it's a good idea to play with the release time to get it to pump in time with your track. Experiment by slowly blending this highly compressed track in underneath your unaffected drums to find the right amount. Generally less is more. Muting it in and out so you hear what's happening will help you get the right balance. Lastly, feeding a touch of bass or a little bit of your main reverb into your smashed drum mix can work very nicely on the right track.
General compresssion tips
• If the gain reduction meter is not returning to zero at all during playback, you are more than likely using too much compression. The signal is just basically being turned down and the compressor is being used as a glorified volume knob!
• Over-compression can sound hugely powerful as an effect, but only if there is balance elsewhere in the mix. If everything is compressed there's nothing for the ear to balance against. The effect will lose its power and as a result the whole mix will sound smaller.
• Always ask yourself why you are compressing. This applies particulartly to electronic music since, due to its heavily sequenced nature, level control is not always required. Plugin makers won't thank us for saying so, but it's true!
• It's a good idea when you are just about to bounce down your final mix to revisit your compression settings and try backing them off a little to see if the overall sound is better. I have often found that mixes breathe more and sound bigger when doing so.
These are just a few ideas to get you started, but remember: there are no real rules. You'll find most professional engineers use compression in completely different ways. The only question that really matters is, do the end results sound good?
Words: Chris Lyth