Dynamic range compression is one of those subjects I feel gets more interesting the more experience you gain using it. Part of this is because of the number of different ways compression can be used. Here are 5 contrasting tasks the humble compressor can be used for in a mix.
Traditional ‘Glue’ Compression
Oh dear we've used the ‘G’ word already! Many people throw around words like glue and punch when talking about compression however what exactly do people mean when they talk about ‘glue’?
The beneficial role a compressor can play in bringing a mix together into a cohesive whole relies on a side-effect of placing a compressor across a bus carrying multiple tracks. The difference between compressing tracks individually and then summing the output of those compressed tracks compared to summing them together and using a single compressor across this summed output is fundamental to this idea of ‘glue’.
I've never been keen on the word glue I prefer ‘interdependence’ by which I mean that events in one track which triggers compression has an effect on all the other tracks by virtue of the audio of all of them being combined before they hit the compressor. Sometimes this can be counter-productive and features such as side-chain filters and multiband compressors have been created largely to counter this effect, but the beneficial effect of this interdependence is a movement within what otherwise might be a rather static mix, and when done well this can enhance the mix introducing a quality of ‘belonging together’. Some people refer to it as ‘sounding like a record’.
In terms of how to set the compressor up a low ratio and relatively high threshold is important as only a small amount of gain reduction is desirable often as little as one dB of gain reduction. The attack time can be used to manipulate the transient of the peakiest part of the mix which in many cases is the snare and the release time needs adjusting carefully as when done well it can benefit the feel and timing of the track but when done badly it can work against it to some extent.
While many people favour specific compressors in this role, for example the famous SSL bus compressor or valve compressors like the Manley Vari Mu, the beneficial, cohesive quality of this style of compression can be achieved with any compressor. The most important thing is not to overdo it
Punchy Parallel Compression
If you ever feel like you enjoy the excitement heavy compression can bring to sounds, but when you bring in enough compression for things to get interesting the compression which was making things sound big and impressive starts to make things sound smaller, then parallel compression may well be what you need. By mixing in a compressed signal with the unprocessed signal (i.e. in parallel with the unprocessed signal) it's easy to get the beneficial aspects of heavy compression, namely energy, density and low-level detail without compromising the transients which bring punch and articulation to the audio. This technique is extremely popular on drums, which can tolerate extreme compression if that's what you want, but it can be used more subtly and rather than creating wild pumping effects it can introduce warmth and intimacy if a little restraint is exercised.
The traditional way to set up parallel compression is by using auxiliary sends routing to a return channel on which the compressor is instantiated. The proliferation of mix controls on compressor plug-ins largely makes this use of sends unnecessary though it still can be very useful as it offers flexibility during the mix.
Any compressor can be used in parallel, but for very aggressive treatments such as those popular on drums, the fastest compressor you have will be a good choice. The 1176 is of course very popular in this application but any compressor can be used and it's definitely worth experimenting, check out Chris Lord-Alge’s use of parallel Neve compressors in the video above. Because it is being used in parallel you have freedom to use far more compression than would be appropriate normally although that doesn't mean that extreme compression is mandatory or even appropriate. Everyone should try this but try not to get carried away!
Subtle Vocal Leveling
Perhaps we should have started here because when explaining what a compressor is this is probably how I would begin. Compressors are after all an automatic volume control and if things get too loud they get turned down. The oldest form of compressor is a finger on a fader turning down loud phrases to even out the difference in level between the soft and loud. People respond relatively slowly compared to machines and when levelling vocals it is on this longer timescale that we are operating. A super-fast compressor plug-in might well be able to respond within a millisecond but isn't what levelling is trying to achieve. The action of the attack and release of a compressor set up to level a vocal will be operating more slowly, the advantage of which is that a slow compression action leaves the internal balance of words intact but evens out the relative levels of entire words and phrases.
A compressor with a very soft knee and long attack and release times does a good job of levelling a vocal but there are other options available today. Dedicated leveller plug-ins exist an excellent example is the leveler section of Sound Radix’s POWAIR, which unlike a compressor plug-in is loudness-based as opposed to the more traditional level measurements used by conventional compressors. Using automation instead of dynamics processing for levelling up can give great results but is something which can be labour-intensive and there are plug-ins available which rather than using compression, instead write automation which can subsequently be edited as desired. Waves make their Vocal Rider plug-in. My preference is for Quiet Arts Waverider Tg. A combination approach can of course be taken where manual fader rides written in your DAW automation are combined with slow gentle compression. It's often a better approach than relying on compression alone.
Vintage Opto-Compression
Optical compression is a great place to start if you are new to compressors. I think of them as ‘do no harm compression’ because they are so difficult to get wrong. Optical compression has been around for a really long time. While in hardware the compressor action was governed by optical components, in software it is the benign characteristics of the compression action these hardware designs exhibited which persists. There are no lightbulbs tucked away in your plug-in.
While some optical compressors offer manual control of attack and release optical compressors typically have only two controls - gain reduction and make up gain. The forgiving character of optical compression means that on sources that suit it, having just two controls is enough and the vintage character of optical compression works beautifully on material which tends towards a sustained character as opposed to being transient rich, though an optical compressor can work brilliantly on drums, it just depends what you’re trying to achieve. Typical applications include vocals, bass guitar and piano though they are more flexible than this rather short list would suggest.
Some extremely well-known classic compressors include the famous LA2A, its counterpart the LA3A, which is particularly favoured for electric guitars because of its more aggressive character. But ‘Optical’ has become a style as much as a way of achieving compression in hardware. In hardware the Emprical Labs Distressor has an ‘optical’ mode, though it’s very much a VCA compressor and I’d always assumed the Summit Audio TL100A was an optical compressor but while it contains a ‘compression cell’, whether it is actually optical seems difficult to establish. But the point is that optical compression is a style of compression. Simple, slow and very hard to get wrong. It is the antidote to agonising over compression settings and everyone should use it somewhere!
Transparent Multiband Compression
So much of compression is about getting a ‘sound’, yet until comparatively recently compressors were exclusively designed with transparency as their priority. The inevitable side-effects of compression become more difficult to deal with as audio becomes more complex and on submixes and busses single band compression can reach its limits if it is going to remain transparent.
Splitting the compression action across several bands is the ideal solution for truly transparent compression of complex signals. Multiband compressors can be used as tone shaping tools, fulfilling the role of dynamic equaliser just as easily as functioning as a compression device but if used simply, with the controls linked across all bands and a moderate amount of compression applied, the effect can be free of the tell-tale pumping of a single band compressor. Multiband compression is a big subject and for a wider perspective on its use check out my recent article Multiband Compression - Handle With Care?.
Conclusion
There are many other ways in which to use a compressor and the intention of this article isn't to attempt to cover all of them. For example you might be wondering why I haven't mentioned sidechain compression? That was a deliberate choice as sidechain compression receives a great deal of attention online although I have to confess I feel the prominence it receives doesn't seem in proportion to the amount of use it gets in mixes outside of some specific genres. Maybe that's just me but the approaches detailed in this article are, with the possible exception of multiband compression, styles of compression I use a great deal. In my recent article on multiband compression mentioned above I discuss how little I use multiband compression though in truth that position has since changed slightly, largely as a result of the writing of that article and some subsequent conversations I've had with fellow mixers.
It's always helpful to step outside of our regular ways of working, although not too much. If you are new to, or have neglected any of these styles of compression recently why not revisit them and see if they can offer something to your current project?