What is Compression?

Matt CookProduction, Tips

Ableton Live's built in compressor

What is Compression?

Compression plays a huge part in every professional mix. Put simply, its roles are most often glue (on busses) and volume control (on individual tracks). We will dive in with a more technical explanation after covering these concepts.

Glue

In the mix, related tracks are in groups, or “busses”. For example, separate tracks for the hi-hat, snare, and kick sounds are grouped into a drum bus. Light compression here helps to create a cohesive sound from different samples, or different microphones in the case of a live mic’ed drum set.

On the master bus, “transparent” (light, non-destructive) compression is used in the same fashion, helping individual tracks and groups to interact with each other instead of just co-existing.

Compression can also be used to subtly or drastically change the sonic character of a single sound – for example, to give the snare more punch, or to give a kick/guitar/anything stronger sustain.

Volume Control

When a musician plays an instrument, volume throughout a take can vary greatly, sometimes to the point that it’s problematic in a mix. Subtle compression is important to even these changes out while retaining musical value that the shifts may carry.

How it Works

When the incoming audio signal rises above the threshold, the audio signal is reduced towards the threshold by the given ratio.

Threshold

The control that determines at what input level the compressor actually starts affecting the audio. If the signal never reaches the threshold, the compressor does not do anything. No audio below the threshold is affected.

Ratio

The strength of compression on audio that rises above the threshold. If the ration is set to 4:1, audio that reaches 4dB above the threshold leaves the compressor at 1dB above the threshold.

An easy way to understand this is to reverse the ratio. With a ratio set to 3:1, audio above the threshold leaves the compressor reduced to 1/3 of its volume above the threshold.

Attack

How quickly the compressor takes effect once the signal rises above the threshold.

Release

The amount of time which the compressor takes to return to normal (or 1:1 ratio) after dropping below the threshold.

Mix/Parallel Compression

Parallel compression is where the incoming dry, unaffected signal is mixed with the outgoing, compressed signal. Labeled as “mix” or “dry/wet”, This allows you to crank up the compression ratio and/or lower the threshold for stronger compression, but lower the mix control so it isn’t as drastic of an effect. Either way is fine and can achieve different results. I prefer to leave the mix at 100%, using lower ratios/higher thresholds. For me, it’s less knobs to worry about.

Side-Chain Compression

Basically, this is controlling the compression/volume of a track with the threshold control routed to a totally different track. When you hear “ducking” of a synth pad when a kick drum hits, this is what’s going on – compression of the synth pad controlled by a threshold routed to the kick drum. This is an advanced concept – watch for a future post on this topic.

How do you approach compression?

Let us know and help your fellow music producers out by dropping a comment below! You can also check out how compression is used in the mastering process in this article.

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