Compression is an often-used processing technique during mixing as well as mastering. It does require a certain amount of experience though, to choose meaningful compression settings and to clearly perceive a compressor’s workings. Because a compressor directly affects the volume of your track, it is very important to bear in mind the psychoacoustic principles discussed in Part 1 of this blog. Therefore, make sure to use the compressor’s make-up gain feature to do an objective before-and-after comparison of your settings. Only then you can accurately judge if the compression actually makes your track sound better, or if it accidently harms your tracks dynamics or transients.
[More info WIKI] There are two main reasons for using a compressor during mastering: to optimize a track’s dynamics, or to add a specific coloration or glue to a track. It’s important to know your reasons for using a compressor in order to choose the appropriate plug-in. For dynamic optimizations, like making the percussive elements in your track sound more powerful, you want to use a relatively clean sounding compressor with full and continuous control over as many features as you can get. This way you can be surgical in your approach, and affect specific elements in your track in a transparent way.
On the other hand, for adding a specific coloration or glue, you want to use a simple compressor that adds as much character as possible to your track. Usually, for this purpose, it’s best to use a compressor plug-in that emulates the sound of one or more well-known analog compressors. It’s important to be conservative while setting such a compressor, as your main goal is to add character without any obvious changes to your track’s dynamics. In general, during the entire mixing and mastering stage, consciously choosing between clean and more characterful compressors can really make a difference.