When used correctly, audio compression can really make a track sound a lot more professional. If it’s not used correctly though, it’s going to adversely effect your mix.
To do the best job you can, it helps to understand what compression is and how to use it – so here’s everything you need to know about Ableton’s compressor effect.
First, the basics.
The function of a compressor is to compress or ‘squash’ the amplitude of an audio track so that it is more consistent. Still confused? This picture demonstrates it pretty well:
As demonstrated by the above image, compressing audio also means that as a whole, your audio can be louder (without any peaking occurring). You might also notice that compression takes away some of the dynamics of the audio – so, if you over compress something it will sound very ‘flat’ dynamically, even though it may sound louder.
Threshold and Output
These form the basis of how a compressor works.
- Threshold – anything above the
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threshold volume will be reduced in amplitude.
- Output – increases the entire volume of the audio.
- The Graph – this displays the result. The horizontal axis is input volume, and the vertical axis is output volume.
So in the below image, everything above -15.6 dB is going to be squashed a little. Use threshold to control how much of the audio is compressed, and then use ‘output’ to bring the volume up as far as you can without the audio peaking.
Attack and Decay
These control what happens when the threshold level is reached.
- Attack – how soon compression kicks in once the threshold level is reached.
- Decay – how long the compression continues after sound drops below the threshold level.
So in the image below, compression will start 1ms after the threshold level is reached. 20ms after volume drops below this level, compression will stop. Attack and decay are necessary for the compression to sound smooth – without them, compression would start and stop too quickly.
These are different ways of determining when the threshold volume is reached.
- Peak – this measures the volume by taking the highest possible point of the audio wave.
- RMS (or root mean square) – this measures the average volume
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over a short space of time. Because it’s an average, some peaks may still sneak through without much alteration from the compressor.
- Opto (optical compression) – this is slightly different again. It emulates a form of analog compression, and in doing so can sound slightly more natural.
Ratio and Knee
These control how harsh the compression is.
- Ratio – when the compressor kicks in, it squashes audio by a certain amount, and this amount is determined by the ratio. Let’s say the ratio is 2 – any sound over the threshold level will be reduced to half the original volume.
- Knee – the graph shown earlier is great for seeing what this does. Instead of a single ‘kink’ in the graph at the threshold volume, a high knee value will smooth the kink out (try it and see for yourself!). Think of it as a way of applying a gradual threshold.
The last few options don’t have as much of an impact on the sound as any of the aforementioned settings do. If you’re not sure what you’re doing, then leaving them set at the defaults is a great option!
If your’e interested though, FF1, FF2 and FB are all different types of compressor
model. ‘Lookahead’ is a setting that controls how far ahead the compressor looks for threshold levels in the audio.
How To Compress
Ok – that’s great, we’ve gone through what everything actually does. But what’s the best way to actually go about compressing something? The following
method works well (by no means the best method though, there’s no such thing!):
- Turn the ratio up quite high, to say, 8.
- Adjust the threshold to a rough value that sounds ok (don’t worry, we’ll change this again later).
- Now, it should be easier to hear the effect that attack and release have. Adjust these to taste.
- Now that attack/release are set, adjust the ratio so that your audio sounds punchy, while still being expressive.
- Re-adjust the threshold to perfection.
- Add a little knee as sounds right.
- Bring up the gain as far as you can, without causing any peaking.
As mentioned earlier, make sure you don’t over compress anything. It will leave your track sounding flat and harsh – it’s worth compromising a little volume for some more expressiveness!