Parallel compression demystified

Compression is a fantastic tool when it comes to taming dynamics. Unfortunately, depending on the compressor used and the degree of compression applied, the processing might change the sound characteristics of your source material a lot. 

Wouldn’t it be great to have a compressor that sounds less aggressive but yet manages to do a good job at reducing the dynamic range? 

Oh yes it would! Enter parallel compression.

Definition: Parallel compression mixes a clean signal with a highly compressed version of the same signal.


In the above picture you see two waveforms: the original audio and a highly compressed version. Observe that in the compressed audio, the transients have been crushed down a lot.

Let’s mix those two signals together!

Now do a thought experiment and think about what happens when you mix those two waveforms together. Two scenarios are possible:

1st scenario: Areas with high volume and transients (the loud parts)

During the loud parts of your audio the main contributing factor will be the uncompressed signal. Why? Because the gain of the loud parts in the compressed audio has been drastically reduced and compared to the original audio its volume is insignificant. I repeat: At points with large transients the sum of compressed and uncompressed signal essentially yields the original (uncompressed) signal.

2nd scenario: Low-volume areas (the quiet parts)

During the quiet parts the compressed signal will contribute significantly to the audio. This happens because we configured our compressor to really squash the transients but not to affect the quiet bits that muchAgain: During quiet passages the volume of the compressed signal is similar to the volume of the uncompressed original. Therefore, the compressed signal will contribute significantly to the volume at these points, thus increasing the volume.

The result

Summing up:

  • We have no gain contribution from the compressed signal during loud passages (no significant volume increase).
  • We have a significant contribution from the compressed signal during quiet passages (significant volume increase).

The result is an increase in volume of the quiet passages without (much) alteration of the loud passages. Have a look at the picture below showing the waveform of the original audio and the waveform after applying parallel compression.

Parallel compression-example-original-and-parallel compression 720x420

Why is this less intrusive than normal compression?

The main difference between normal and parallel compression is that

  • normal compression reduces dynamics by squashing the peaks.
  • parallel compression reduces dynamics by increasing the volume of the quiet parts.

As it happens our brain is trained to pay a lot of attention to transients since these can often be attributed to imminent danger (think of a car crash). Therefore, our brain picks up any alteration of the sound quality of a transient (like kick or snare drum) quite easily. Conversely, altering the quiet passages in between transients often passes unnoticed and thus sounds more natural.

Let’s have a listen!

In the MP3 below you’ll hear a drum loop. First the original, then with parallel compression applied. I’ll cycle a couple of times between original and parallel compression.

Wow, that was a trip… I hope you liked this introduction to parallel compression! If you want to read more, there is another post in which I explain how to set up and tweak the parameters of parallel compression. You’ll find it here

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That’s all folks, remember to make some noise!

Posted in Compression, Mixing and tagged , , .