Compression is the most important but oft-undervalued mix process. Here we detail the basics, some famous compression techniques and those greatest of hardware and software compressors…

Though it’s a frequently discussed topic, we’ve lost count of the number of people who’ve turned to us and asked, “just what does compression do in music?”. Let’s answer that question once and for all. Compression can be used for a wide range of applications, from evening out the volume of audio signals, through adding warmth and character, to enhancing audio so that it bites or punches through a mix. But all of these share the core compression philosophy, that of controlling the dynamic range of an audio signal.

WHAT IS COMPRESSION AND WHAT DOES IT DO?

Take any instrument or voice – or any sound you like, in fact – and it has a range of levels, from its most gentle, quiet tones to its loudest and most extreme. This, is referred to as the dynamic range, and the first main use of compression is all about taming this out to a manageable mix level so that it plays its part but not at the expense of anything else.

At a very basic level you are simply making the loud parts more quiet so that you get a more consistent level and you can then raise the gain of the overall audio, if you wish, so that the track is not just more even, but louder in the mix. Job done!

However, you don’t want to flatten an instrument or vocal out completely, each sound has a natural dynamic range which defines its character and emotion. But, nor do you want it to be all over the place; a bass guitar part loud one note and quiet the next, a vocal shining through on one line, only to back off tentatively on the next. Good compression is all about maintaining a good and characterful sound, but also managing that range so that everything in you mix can shine… when you want it to.

Then there’s the second use: compression employed more as an effect. Here you might want to add character. Early compressors imparted their own sound or colour, and there are many plugins that emulate that feel today. Or you might want to go a tad extreme.

Tightening drum loop transients to give you a punchier sound or increasing the sustain on a guitar, is bread-and-butter compression but more extreme applications include crushing the sound for lo-fi excellence.

Compressors can also be used more as effects by way of different techniques: side-chain, multi-band and parallel are just three techniques where compressors can be used as dramatic effects.

So, the answer to the questions, ‘what does compression do in music’ can have multiple answers.  Before we look at some of the best classic and new compressors, and just what compression does in music, let’s go back in time and look at a little compression theory.

HANDS-ON WITH COMPRESSION IN MUSIC

Before the era of alluring visualisations that we’re used to in today’s plugins, compression was done manually. The most obvious form or manual compression is with a technique that is still employed today for singers when they move the microphone away during a loud passage of singing, and then bring it close in for a more intimate verse. It works and is still an especially great technique when used live (and on TV talent shows it makes you look like you know what you’re doing).

A similar technique can be applied in the studio. Got some recordings or input signals with varying volumes? Well, try riding the fader; this will allow you to manually control the dynamic and some say that this gives you a much more natural sound. And with modern software DAWs allowing you to automate volume, surly this is the way forward and no further compression is needed, right?

Well, a complicated mix demands a lot of fader riding and would require lengthy volume automation when it gets down to the nitty gritty of sonic movement within a track.

A more convenient way is therefore needed, simply to manage the more intricate micro-dynamic movements within a track or an overall song, and this is where compressors came in to rescue the day – and the dynamic. Applying compression in a live context can be a little different, however.

If you can get to know how a compressor affects an audio signal to even it out, then you can set one up in no time and riding the faders, moving the mic and lengthy gain automation will be superseded. But first you’ll need to understand the basic controls of a compressor.

WHAT DOES COMPRESSION DO IN MUSIC?

First up, let’s run through the core controls. The threshold is simply the level on a compressor that you can set before it actually starts working. Set your threshold level to a certain level and signal processing will be applied over this level.

The ratio is the amount of compression applied to a signal once is crosses the threshold. Set it low at 2:1 for gentle compression which means that if the signal goes over the threshold by 2dB it will be halved to 1dB. Set it at 5:1, and you will have to go 5dB over the threshold to get 1dB out so this is a harder compression. You can go for a much higher or aggressive ratio so that when the ratio is set at 20:1, for example, virtually everything is compressed above the threshold and the process become ‘limiting’. Higher ratios are useful if you have odd peaks that need compressing whereas more gentle compression is good for, say, a vocal where the variations in the dynamic range are less dramatic.

While we’re talking threshold in relation to compression and music, we should quickly mention the ‘knee’, aka, the shape of compression action on the audio once it has passed the threshold. Hard knee compressors do it faster while soft knee are less abrupt and more musical because of it.

The attack and release values are essentially the timing values for how long it takes the compressor to kick in or stop; its reaction time or how long it takes to control the volume. The attack value is how fast the compressor gets to work when the signal exceeds the threshold.

A fast (short) attack time means the compression kicks in quickly, reducing the transient of the sound, smoothing it out and increasing sustain, and is good for dealing with clipping signals. Conversely a slower (long) attack time means compression kicks in slowly, allowing transients through for more punch. On a vocal, for example, a slower attack time might make the start of certain words sound clearer in a mix whereas a short attack time will make them more muffled.

The release time is about how quickly the compressor stops acting upon the signal once it falls back beneath the threshold. In other words; how quickly the gain goes back up after the compression has been applied. A fast (short) release time means the compressor stops faster so the signal can be more dynamic but set too fast can result in a pumping effect or a choppy vocal. If the release time is slower (long) the compression is still being applied resulting in a smoother sound but too slow and the result might be sluggish and just have a perceived lower volume.

Finally, the make-up gain level at the end of the signal flow allows you to ‘make up’ for any losses incurred during the compression process by boosting the overall level.

DIFFERENT COMPRESSOR TYPES

There are a good half dozen different types of compressor but the main ones include the following: FET compressors utilise Field Effect Transistors and are known for being fast – the 1176 being the classic (see below), although they are generally not as good for the widest range of uses, nor are they transparent.

That said, an 1176 employed in the right hands on drums, vocals, bass and keyboards will give you great results.

Diode Bridge compressors, those by Neve being prime examples, offer more flexibility in the compression curve and a more distinctive tone.

Optical or Opto compressors (see the LA-2A below) use an optical cell to attenuate the output signal level. They can be slower than other compressor types but are well regarded for vocals and overall mix bus processing.

Tube compressors like the Fairchild 670 tend to be slower in terms of attack and release times so offer more colour or a ‘vintage’ sound.

Finally, VCA (Voltage Controlled Amplifier) compressors, like the iconic SSL G Series and dbx 160, can be completely transparent so are great for every stage of the production process, from tracking to mastering. They are also especially great for drums and sorting out dramatic peaks in an audio signal.

WHAT ARE SOME GREAT MODERN COMPRESSORS?

Up to now you can probably see that the market for compressors in music production, and a compressed ‘sound’ has been dominated by early, vintage models. This means that many newer software (and indeed hardware) compressors emulate this classic sound – indeed we look at three of the most iconic compressors below and there is a huge market of both software and hardware emulations surrounding each.

However, there are also newer compression techniques that have spawned their own plugins. Parallel or New York compression had its peak time in the 90s, and is a technique that can be used with compressors including the classic dox 160 and 1176 – not to mention one that is easy to set up within your DAW. So in terms of the tools that can help you understand what does compression do in music, this range of plugin developers have leapt on the sound to produce plugins specific to its process – which gives you more control over wet and dry channels in the signal chain – or which make the process easy to implement.

Some great examples include Arturia Comp Tube-Sta, Baby Audio Parallel Aggressor and I Heart NY, FabFilter’s Pro-C 2 and Boz Digital Labs Manic Compressor.

Sidechain compression is also another well-used technique (think ducking kick drums – you know the sound!) with newer developers supplying many a title even though – again – you can set it up with your stock DAW plugins relatively easily.

Good plugins to turn to for the pump include Blue Cat MB-05, Cableguys Shaperbox 2, Pulsar Audio Mu, Softube CL1B, Sonible smart:comp and Xfer Records LFO Tool.

Multiband compression is, of course, exactly what you think it is, allowing you to focus in on specific frequency bands. This is great for mastering or lead vocals where you want to home in on very detailed areas when applying compression in music.

Some great compressors for this include FabFilter’s Pro-MB, IK Multimedia T-Racks Quad Comp, McDSP ML4000, PSP Audioware VintageWarmer 2 and Vengeance Sound Multiband Compressor.

Finally, there’s ‘look ahead’ compression. Some super-fast compressors deliver their compression quickly but with unnatural results.

Look ahead compression allows you to apply smoother and more natural compression fast, by starting the process early and in anticipation of an incoming signal. You can achieve this simply by duplicating an audio track, and placing one copy early and applying its bus compression to both tracks (mute the early track) so that the later track gets the early compression. However, some compressors have this feature built in and your DAW delay compensation means you don’t have to fiddle with our technique! Great examples include FabFilter Pro-C 2 and Pulsar Audio Mu and many sidechain compressors will allow you to do this with ease.

So have we answered the question of just what does compression do in music? The best way to check is to get hands on and apply this knowledge yourself. That’s our sprint over the basics of compression, compressor types and just a smattering of the techniques you can employ. Now for our top three compressor classics!

THE TOP THREE CLASSIC COMPRESSORS IN MUSIC PRODUCTION

Fairchild 660/670

Original units of these valve limiters have become so rare (less than a thousand were produced) that you can expect to pay up to £45k (at least there’s one on sale on Reverb for that price).

The original, designed by Rein Narma and licensed by Sherman Fairchild, made its name as a mono compressor for broadcast but has since become a studio recording icon – thanks largely to Geoff Emerick using it on the Beatles’ vocals and Ringo’s drums. There are countless hardware and software recreations.

Hardware approximations and recreations: Amtec Audio Model 099, Analogue Tube AT-101, Anamod Audio AnaMod 670, IGS Audio Tubecore, Pendulum Audio ES-8, Undertone Audio UnFairchild 670M II.



Software emulations: IK Multimedia Vintage Compressor, Overtone DSP FC70, Universal Audio UAD Fairchild Collection, Waves PuigChild 660/670 (pictured).

Teletronix LA-2A

No answer to the question of what does compression do in music can be truly complete without a mention of Bill Putnam. His is a name all over the history of famous compressors. In the 60s Putnam senior bought into the optical tube-based compression technology developed by Teletronix to continue production of the LA-2A and in 1999 Putnam junior bought the unit back after a 30-year break, and it is still being made to this day.

Celebrated by just about every engineer the world over for its sound – which can be gentle and transparent or powerfully colourful – it has been on records by everyone from Daft Punk to Nirvana.

Hardware approximations and recreations: Klark Teknik KT-2A, Tube-tech CL 2A, Universal Audio LA2A, Warm Audio WA-2A.

Software emulations: IK Multimedia White 2A, Native Instruments VC 2A by Softube, UA LA-2A Classic Leveler Collection, Waves CLA-2A.1

Urei 1176

In 1966 UREI (United Recording Electronics Industries) founder Bill Putnam designed the FET transistor-based 1176 Limiting Amplifier. It became a classic thanks to a wide range of sounds and applications – it sounds great on drums, or vocals and bass and is capable or powerful grit and distortion thanks to its famous ‘all buttons in’ mode.

It made a huge impact in studio recording history – think Zeppelin and the Stones – and now you can get many modern offerings including Universal Audio’s 1176LN, essentially a reproduction of the original.

Hardware approximations and recreations: Klark Teknik 1176-KT Black Lion Audio Bluey, Lindell Audio 17XS, Universal Audio 1176LN or 6176, Warm Audio WA76,

Software Emulations: IK Multimedia Black 76, Native Instruments VC 76, Slate Digital FG-116, Softube FET Compressor, Universal Audio 1176 collection, Waves CLA-76.