What is a Compressor?

Learn in plain english what an audio compressor is

Have you ever wondered what an audio Compressor actually is?   

Well, you’ve reached the right place to learn more about what a compressor is and what does it do.

BUT first…  let’s just explain that this isn’t the scientific definition of what an audio compressor is.  It’s the down to earth, plain speaking, real world explanation for any beginner looking for the answer to this commonly asked question.

The Definition

A compressors primary job is to reduce the audio level if it reaches a certain level, that level is called the threshold.

The amount is it reduced by is called the ratio.

This is not the same as changing the volume as the compressor only affects the audio which is above the threshold

Some compressors allow you to alter the threshold and ratio and some don’t.  It’s all down on the compressor you’re using.

Compressor features vary from unit to plugin but here are the most common features you will see.

Attack, Release, Knee & Gain

You may also see controls such as attack and release. 

The attack changes how quickly the audio is affected when it hits the threshold.

The release changes how quickly the threshold returns back not effecting the sound.

Most compressors include gain controls which means volume.

These may be located:

  • Before the compressor – Input Gain – which allows you to boost or reduce the volume of sound going into the compressor. 
  • After the compressor – Output Gain – this allows you to boost or reduce the volume once the compressor has effected the sound

Typically you may want to boost the output gain up by the same amount it’s being compressed by.

For example, if you can see the compressor is reducing the sound by 4dB then your would look to boost the output by 4dB to help maintain the same level.

Finally, some compressors have a knee parameters.  A knee is how the compressor transitions between the non-compressed and compressed sound.  This could be a hard or soft knee to determine if the compression has a sharp or gradual effect.

Types of Compressors

Single Band Compressors:

A single band compressor affects the full frequency of the sound.  There are many types of single band compressors which affect the sound in different ways.  Some may have different attack and release times and some may saturate the signal more. 

The main ones are:  Tube, Optical, FET & VCA.

Multi Band Compressors:

A multi-band compressor allows you to split the sound into frequencies and compress the different bands individually.  For example:  If you want to control the high frequencies of a vocal recording but not the lows then a multi band compressor will allow you to.  Most multi bands compressors will also allow you to have different attack, release and gain parameters for each band.

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