What Is A Compressor?

The Compressor is one of the most misunderstood effects of all – here’s what it does, and why you might need one

What Is A Compressor?

Imagine the scene- two guitar lovers are trying out some effects in their local store. Firstly, they kick in a reverb, and the guitar tone disappears into a cathedral of echoes. ‘Cool!’, they chime. ‘That’s really spacious…’

They switch out the reverb, and next, a flanger is tested. The tone becomes a mass of whooshing, spacey madness. ‘Nice,’ they say, ‘very Hendrix!’

This is switched off, and they kick in the final pedal: a compressor. ‘Hmmm…’ they say, ‘that doesn’t seem to do much. Maybe it’s broken…’

This how one of the most essential effects gets overlooked by guitarists, and the situation is often similar with those taking their first steps in home recording or production, too.

Because it isn’t immediately obvious what it does, and its audible effect is usually not dramatic, the compressor remains a bit of a mystery to some. And yet, this effect is key to achieving truly professional sounds, or getting particular guitar tones.

So what is a compressor?

What Is A Compressor?

Put simply, a compressor reduces dynamic range.

The dynamic range is the range from the quietest point to the loudest point in a given piece of audio. So, if that was, say, a guitar performance, that would most likely be from silence up to the loudest bit of the performance.

What Is A Compressor?

How does it work?

A compressor works by using a (usually adjustable) threshold. When the signal level running through the effect is below this threshold, it is unaffected. When it goes above the threshold, the effect is triggered.

As the signal crosses the threshold, it is reduced by a (usually) pre-set ratio, ‘squashing’ the peaks.

What Is A Compressor? - Uncompressed Wave Showing ThresholdFor example in the (albeit rather crude) diagram above, it shows ½ of a sine wave, uncompressed. In the diagram below, it shows roughly what you could expect to happen to the peaks above the threshold if the compression ratio was set to around 2:1.

What Is A Compressor? - Compressed Wave

The typical controls of a compressor are:

  • Threshold – this dictates the level at which the compressor will start to compress. Set high, it will just reduce the peaks. Set low, it will reduce the level of nearly everything.
  • Ratio – the amount that the signal above threshold will be reduced. Typically displayed as X:1, where X is the level above threshold. So, a 2:1 ratio reduces 2db to 1db, 4:1 will reduce a 4db peak to 1 db, etc. If the ratio is infinity:1, this is known as limiting, and completely flattens any peaks.
  • Attack – this controls the length of time taken for the effect to reach full compression. Because all sounds are different, to make compression less obvious, and more natural, the attack can be set that the effect smoothly transitions to compression.
  • Release – this is the time it takes for the signal to return to normality from full compression after it drops below the threshold. This is tweaked for the same reasons as attack time.

What Is A Compressor? - Cakewalk RE-2A Levelling Compressor - Reason Rack Extension

Why use it?

One of the main reasons for using a compressor is to make recorded signals in a studio environment more even. So, if a peak vocal performance is making mixing difficult, they can be easily tamed with a compressor.

They are also generally employed over a full mix, as part of the mastering process. This can be to reduce the dynamic range such that it will comfortably fit within the dynamic range of, say, a CD, without being too quiet or clipping.

Another common use is to ‘duck’ sounds, which muddy others, most commonly, bass and kick drum. The compressor is set such that it compresses the bass, but only when the kick drum is played.

What Is A Compressor? - MXR Dynacomp

Compressors can also be used as an effect, to achieve a particular sound. They can give kick drums more ‘punch’ , for example.

By setting the attack to a reasonably short time when used with guitars, for example, they can produce a very percussive sound, as the compressor just misses the initial attack of the guitar – perfect for choppy, funk sounds.

There are innumerable compressors available in software plug-in form, alongside plenty of guitar stomp-boxes, too. Take a look at the stomp-boxes here.

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Joe is a contributor for the Dawsons Music blog. Specialising in product reviews and crafting content to help and inspire musicians of all musical backgrounds.