Compression: A Misunderstood “Black Magic” Explained

A simple guide to compression and how it works

To understand compression first we must understand what it means to compress in Audio Engineering. Compression either amplifies quiet sounds or reduces the volume of louder sounds effectively narrowing an audio signal’s dynamic range. For some people this pops up a red flag immediately because they have the concept that the more dynamic range you have the better your sound recording. While it’s true that over compression can cause clipping and sometimes unfavorable distortion, when done tastefully and proper, compression can really make a mix stand out and feel more balanced. But before we get to that let’s first understand the important facets of compression.

classiccompressor

Figure 1: A Classic Compressor courtesy of KVRaudio.com

Threshold: Threshold is measured in decibels (db) and is the level at which the compressor actually starts to take effect. A compressor set with a higher threshold of -5 db is going to treat less of the signal as opposed to a compressor set with a lower threshold of -30 db.

Ratio: Ratio tells us how much compression is actually being applied to the signal. A ratio of 2:1 means that if the input of the signal is 2 db’s over the threshold, the signal’s output is actually going to be only 1 db over the threshold. This equates to a 1 db of gain reduction.

Attack/Release: Attack and release have to do with how long it takes for the compressor to start and finish compressing the signal after it crosses the threshold. The timing for both is measured in millisecond (ms).

Hard/Soft Knee: This controls whether the response after the signal crosses the threshold is gradual (soft knee) or more abrupt (hard knee).

Gain Makeup (Output Level): This is usually at the end of the chain because this can be used to add a fixable amount of gain to bring back level loss after compression.

After understanding the fundamentals how can compression be put into practice?

So now that the basic fundamentals about compression are understood we need to look at when compression can be a solution to fixing an audio track. One easy example is a singer that moves away from the microphone too much making it hard to have proper levels and let it properly sit in the track. By using a compressor you can effectively narrow the dynamic range of the singers track and making the varying volume changes less apparent. By doing so this will allow the singer to sit better in the mix and can prevent them from being drowned out when the backing track gets louder.

waveforms

Figure 2: Compression Before & After courtesy of Articulate.com

Now that you have a simple understand of compression and how it works what are some other possible times when you may need to use compression and why would it be the right choice?

 

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