Understanding how compression works in recorded music

From Resident Advisor:

RA’s Jono Buchanan simplifies one of the most misunderstood of production processes.

In this tutorial, we’ll be looking at all things compression. Along with EQ and reverb, compression is one of the most common types of processor used around recording studios. To understand why this process is so widely used, we need to understand what compressors do, what their parameters are and how the role they play can be useful to individual sounds, groups of sounds and even to entire mixes.

A compressor’s role is to reduce the dynamic (volume) range of the audio file or instrument onto which one is inserted, effectively lessening the volume gap between a part’s quietest and loudest moments. It’s easiest to think about this when a compressor is applied to a long, dynamic part such as a lead vocal, though of course an individual drum sound has a dynamic range too—these start with a loud initial hit and fade to silence, so drums have a wide dynamic range, it’s just that this range plays out over a short period of time. We’ll see how compressors can help shape drum sounds a little later, but first let’s see what they do for longer parts with less predictable dynamics.

To understand how compressors work, it’s worth analyzing their key parameters to understand how these combine to reduce dynamic range. In this picture, you can see how volume rises if a sound starts from silence and grows to maximum volume along a linear path.

The first parameter common to all compressors is Threshold which sets the point above which dynamic change will occur. By placing the Threshold point in the middle of this volume rise, the quieter section of the sound will remain uncompressed, while the upper section will have its dynamics compressed.

The extent to which the level of the audio above the Threshold Point is reduced in volume is set by the Ratio. A Ratio of 2:1 will ensure that the volume above the Threshold only grows to half its original volume, whereas a Ratio of 4:1 will reduce the volume to a quarter of its original. The greater the Ratio “number,” the greater the volume reduction applied to the compressed section of the audio file, as you can see here.

What this means is that the overall dynamic range of the sound changes. Whereas the original dynamic range is “total,” the new dynamic ranges are smaller. If you set the Threshold point in the middle of a linear volume rise like this and set a ratio of 2:1, the new dynamic range will be 75% of the original. If you set a ratio of 4:1 it will be 62.5% of the original.

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