Every track needs a compressor, and finding the right way to apply it needn’t be a struggle. Rob Boffard demystifies this vital and often misunderstood effect…
There is no other technique more likely to make your music sound amazing than compression – if it’s used correctly. Compression is why songs on the radio sound so amazingly full and rich. It’s why drums are tight and punchy, why you can pick up vocals in in mix, why you can take any piece of audio you like and turn it into a growling monster. More than any other audio effect, compression will make you sound like a total pro.
That doesn’t mean it’s easy. EQ, which we dealt with last time, is similarly powerful, but we like in that particular effects to Tetris: easy to figure out, hard to master. Compression isn’t quite the same. It’s reasonably tricky to use at first, especially if you’ve never had a crack at it before, but once you figure it out – and see what it can do – you’ll have no trouble implementing it in your tracks.
Pump it Up
At its most basic level, compression is about evening out the volume in your tracks. Whenever you record a song, there will be some things that are louder than others. It’s perfectly possible to make a mix from these untouched sounds, but you may find that some of them vanish into the mix simply because they’re too quiet. When something has compression applied to it, it makes the loud bits softer and the soft bits louder. When everything is consistently compressed, the sound will appear to be universally louder after the volume is ranged.
You can actually see this visually. A piece of audio that hasn’t been compressed will be made of spiky waveforms, with plenty of peaks and valleys. Compress that audio, and those peaks and valleys will ‘squash’. The resulting shape will look more like a sausage, with a flat, even top and bottom.
The basic anatomy of a compressor, including ratio, threshold, and make-up controls
Most DAWs will have more than one compressor plug-in included. The easiest way to show the power of compression is to actually do it, so load up a piece of audio and slap on a compressor as an insert effect. You will immediately see a bunch of controls. Chances are, these won’t make sense right away, but give us a few minutes and we’ll demystify things. If you can, try load up a compressor with the visual readout, not just a bunch of faders.
The first thing to pay attention to is the threshold control. Essentially, this sets the point at which the compressor kicks in. If you lower this to, say, -20dB, then the compressor will activate when ever the sound goes above that level. With us so far? Good. Now were going to control what happens after the compressor kicks in.
This control is called the ratio. It tells the compressor how much audio to actually let past the threshold. It’s called the ratio control because it literally expresses this as a ratio: if you set it at a low ratio, such as 2:1, then every 2dB would be pushed back down to only 1dB over the threshhold. 2:1 is actually quite gentle; if you wanted the compression to be more noticeable or powerful, you’d set it at a much higher ratio, like 10:1, which means that every 10dB would be reduced back down to 1dB. At a ratio of 1:1, nothing happens and there is no compression.
Again, the easiest way to hear what this will do to your sound is to try it. A good compressor will actually include a visual feedback to show you how much your sound is being reduced by, which can make it easier to get consistent compression.
There are a couple more controls you need to pay attention to–primarily, attack and release. It’s very easy to make a compressor act like an on/off switch, so that the second you go above the threshold the compressor immediately steps in to kill the sound. But sometimes, this approach can make things sound a little unnatural and forced. The attack and release controls act like a buffer, stopping the compressor from immediately activating, and once it is activated, stopping it from turning off right away.
These controls are usually measured in milliseconds, and you shouldn’t have to increase them too much to start hearing the difference. An attacker value of 2.5 ms, for example, means that the compressor will only kick in 2.5 ms after the threshold is crossed. That means the sound has a chance to go a little bit over limit before it’s pulled back, resulting in a slightly more natural sound.
Side-chaining can be a very clever way to create more space in your track, and to stop two elements – such as a kick drum and a bassline – getting in each other’s way
Then there’s the make-up gain. When you’ve compressed something, it’s obviously going to be at a low volume – since, you know, you’ve made the soft bits louder and the loud bits softer. The make up gain raises the overall volume of the sound after it’s been compressed.
One or two more – and the good news is, you shouldn’t have to worry too much about these for the time being. They’re just good to know. Some compressors include what is known as an expander. It works like a reverse compressor: whenever the sound drops below a certain threshold, the compressor will stop it getting too quiet. Also, you may see a control for something called a knee. This smooths out the transition when the compressor kicks in, making your sound even more natural.
Compressors are wonderful. They have an infinite number of uses, and as we said, they can drastically improve your track. We’ll run through a couple of these uses now, to show you why it’s worthwhile getting familiar with what compressors do.
One of the most common uses of a compressor is to ‘catch the peaks’ of the sound. If you use a high threshold and a gentle ratio on a sound, then you can stop it clipping and distorting when it gets particularly loud, which is great for taming sound that doesn’t have an even volume throughout. This kind of compression is almost transparent: you hardly know it’s there, mostly because it’s only activating when the sound gets too loud.
Limiting is a good way to finish off the track, as it stops the master output from clipping
You can also get plenty aggressive with the compressor. If you take a bassline, and hit it with a low threshold, aggressive ratio and low attack and release values, then raise the make-up gain, you’ll end up with an amazingly punchy, growly, in–your–face bass noise. That grungy sound in dubstep? It’s been compressed to the nines.
There are more subtle uses the compression, too. Take a track, then create a parallel version of it. Put some heavy compression on the parallel version–in audio parlance, the track is now ‘wet’. Mix this track in with the ‘dry’ track, and you can get an extremely attractive level of compression: one which combines the qualities of the original sound with the punchiness of the compressed version.
It’s a little too complex to go into here, but there’s a wonderful technique called side-chaining. With this technique, you can lower the volume of the sound when another sound kicks in. If you’ve got a baseline, then you can have it automatically compress whenever a kick drum plays. The pumping sound in most dance records is because of aggressive side chaining. Once you’ve got this technique down, you’ll find yourself using it again and again.
Compression isn’t just used for mixing. It’s almost always used to master a track as well. One of the major ways that mastering engineers apply a compressor is as a limiter.
A very high ratio – anything above 20:1 – will effectively stop any and all audio from crossing the threshold. If you put a compressor on the master out, and keep the threshold nice and high – around -3dB – then your track will never go above that particular level. As long as you’ve mixed things properly, this is great for adding fullness and shine at the very end.
Fat as Butter
Don’t worry if you don’t get compression straightaway. Like we said at the very beginning, it can sometimes be a little hard to wrap your head around. We don’t mean this in a condescending way, either; compression is notorious for giving producers hell. The best way to figure out how it works and how you can use it is to sit down and play with it.
That being said, there are definitely some principles you want to bear in mind. Try not to overdo the compression. Like many effects, it’s at its best when working quietly in the background. If you’re going to use aggressive compression, save it for one or two elements in your track.
Finally: not all compressors are created equal. There’s a reason why some can run for hundreds or even thousands of pounds. Vintage compressors will often impart a characteristic warmth to the sound, even when a little bit of compression is used. And the more you use compression, the more you’ll start being able to pick up this warm, and the more choosy you’ll be about your compressors. This is a very good thing – for your tracks, if not your bank balance.