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AUDIO COMPRESSOR EXPLAINED IN SIMPLE TERMS

March 22, 2019

BY CJ JACOBSON

Alright, lets first start out by explaining what an audio compressor does, before actually explaining what all the settings and knobs do in it.


Things A Compressor Can Do And When You Can Use It:

  1. If you need to level out the volume of an audio track, you would use a compressor.
  2. If you need to add sustain to your guitar, bass or any other instrument, you can use a compressor.
  3. If you need to tame peaks and have total control of the dynamic range of your audio signal, you would use a compressor.
  4. If you want the vocal track or any other instrument track stand out with more depth and punch, you can use a compressor.
  5. If you want to have your bass guitar have a pumping effect, you can use a compressor.

Those are basically some of the instances ware you can use a compressor.


Now lets explain what each knob, slider and setting does on an audio compressor. I'll explain it as simply as i can. If you learn what each setting does on all your effects, then you can dial in any sound you want. 


Threshold - This is ware the actual compression begins and ends. When the audio signal goes above the threshold, it starts compressing it. When it goes below the threshold, it stops compressing it.


Attack - This will determine how fast the compressor starts compressing the signal after it reaches the threshold. This is when it starts to 'attack' and compress the signal. The shorter the attack time, the faster it starts to compress the audio signal.The longer the attack time, the slower it starts to compress it and as a result, some of the signal may not get compressed. 

This setting is very important as to fast of a setting can squeeze the life out of your audio and on the other hand, a slower attack time may not catch all the peaks. So you need to find a balance for each instrument and mix you compress. Slow attack times are good for adding punch to your sound.


Release - This setting controls how fast or slow it takes the compressor to stop compressing when the signal goes below the threshold.

A fast release setting can make the audio signal sound like its breathing or pumping. A slow release can add sustain to your audio, but to slow of a release can cause unnatural artifacts. As always, find the right balance.


Ratio - This determines how much compression occurs when the audio signal passes the threshold setting. The higher the ratio is, the more compression occurs after it passes the threshold.


For example, if you have a ratio of 4:1 and the threshold is at -12dB, and your audio signal goes over the threshold by 4dB's, just 1dB will be allowed to go past it. If your audio goes over the threshold by 8dB, only 2dB's of audio will pass it. If it goes over by 12dB's then 3dB's of audio will pass it.

You can use this formula with a 2:1, 3:1, 5:1 and every other ratio setting.


Are you with me still? If you are not, just know that they are just numbers and set it to what your ears like the best.


Knee Setting - There are 2 types of knee's, hard and soft. 

  1. Soft Knee settings make the compressor apply compression in an increasing amount until it hits the ratio.  It also doesn't wait for the audio to reach the threshold to start compressing. It will get applied gently until it reaches the threshold.  This is why it can make the transformation between the uncompressed and compressed audio signal more effortless and less intrusive.  Soft knee settings are good for guitars, violins, synths and pianos and instruments that need to sound smooth.
  2. Hard Knee Settings make every part of your audio single get compressed by the entire amount of the ratio. It will not apply any compression until the audio reaches and passes the threshold, unlike the soft knee setting. This setting makes the compression more intrusive and less smooth. It good for fast peaks, like drums and percussion instruments

Depending on the compressor, some have just a soft and hard knee setting and some will have numeric number representing how soft or hard the knee is. An Example, A compressor I have has a knee setting of 1 through 30. 30 being the hardest and 1 being the slowest. 


Make Up Gain -  This brings your signal back up after it has been compressed. Because when audio is compressed, it usually lowers the signal, so this brings it back to an appropriate level that is decided by you.


I think that just about covers it. If you have any questions, just ask. If you get confused, just remember, that your ears are the best judge of what sounds good and what doesn't, do not rely on settings things are set to.


CJ Jacobson - Mastering Engineer

audio-mastering-mixing.com