A compressor is a device for automatically controlling the level of an audio signal. Specifically, a compressor reduces the volume of an audio signal when the volume exceeds a threshold set by the user. A limiter is a specialised form of compressor that effectively limits the input signal to the threshold level.

The Controls

Typical controls you will find on a compressor are:

  1. Threshold
    1. Peak / RMS
  2. Ratio
    1. Knee
  3. Attack
  4. Hold
  5. Release
  6. Gain
  7. Bypass


The threshold control is used to set the threshold level. This level is the level, in dB, above which compression will be applied to the input signal.

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Peak / RMS

This setting is used to dictate how the the loudness of the trigger signal is perceived by the compressor.





The amount that the signal is attenuated by is dependent on the selected compression ratio. For example, if the compression ratio is 4:1 and the input signal exceeds the threshold level by 4 dB the output signal will only exceed the threshold by 1 dB. When the signal falls below the threshold no compression will be applied. If the compression ratio is set to 20:1 or more the compressor is being used as what is known as a limiter. This is because the signal is effectively being limited to the threshold level. Most compressors have a broad enough range of compression ratios that they can operate both as compressors and limiters.

Compression Ratios


Attack, Hold and Release

To make a signal sound more natural at the moment of applying or removing compression, i.e. the signal crosses the threshold level, most compressors provide the ability for the user to adjust the attack and release times. In some compressors these controls are automated. Using the attack and release times, the application of the effect can be made less blatant and the transition smoother.









Compression Details

The Knee



Hard Knee Compression



Soft Knee Compression

When would you use Hard Knee?






Side Chain


Compressor Side-chain
Side-chain Circuit, Standard Configuration

Using The Side-chain During Mixing


Side-chain mode
Side-chain Circuit, Monitoring Key Insert







Discuss this article in our Music Forum.


John Moxey

Songstuff Site Crew


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