Compression Basics

A quick and dirty introduction to compression.


What is compression?

Automated volume control.

It’s essentially the same thing as automating a volume lane or simply moving a track fader as the song plays.

With a compressor, it easily gives us the option to do this automatically under certain parameters. We can say “turn it down by this much once the level crosses this certain line”.

How compression works

It’s helpful to think of a compressor serving two functions:

  • Automated volume control
  • Overall gain increase

The first function merely reduces any volume that passes the threshold. Perceptually this sounds like a gain reduction, making the track seem quieter. The purpose of this first stage is to provide us with more headroom.

The second stage, sometimes called “output gain” or simply “auto” takes the leveled out waveform from the previous stage and increases the overall gain, making it perceptually louder.

Looking at the whole process, we’re simply reducing the highest volume spikes in relation to the quieter sections, then boosting the whole signal.

Why compression?

Evening out the levels in a performance or recording. Reducing the dynamic range between the quietest and loudest portions helps create a consistent level and can make listening more enjoyable if done correctly.

Subtle and “clean” compression can be seen as a utility, whereas more aggressive or “colored” compression can be used as an effect. The former of the two can be more difficult to achieve, as its very easy to over-compress something into the “colored” spectrum. Heavy compression can be compared to saturation or overdrive as the signal is pushed, flattening out the waveform.

Typical compressor controls

Your compressor of choice may have different features or capabilities, but the following are typical controls found on most compressors available currently.

Threshold

This is the level (in dB) where the compressor starts to apply gain reduction.

Ratio

The amount of gain reduction applied once the threshold is crossed.

For instance, with a ratio of 4:1 we see 1dB remaining for every 4dB over the threshold. Ratios over 20:1 essentially function as a limiter.

Attack

How quickly the compressor applies gain reduction.

A slower attack rate will allow initial transients to pass through unaffected by the compressor, giving a more natural sounding result. A faster attack will clamp down more on the transient, giving a more “squashed” sound.

Release

The amount of time until gain reduction recovers and can be triggered again by the threshold

Final Thoughts

This is in no way a comprehensive guide, there are still plenty of other functions to discover within compression, such as hard and soft knees, filtered sidechains, mid/side compression and much more.

Don’t be afraid to experiment! Use your ears and your discretion.


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