Compressors and limiters are specialized amplifiers used to reduce dynamic range — the span between the softest and loudest sounds. The use of compressors can make recordings and live mixes sound more polished by controlling maximum levels and maintaining higher average loudness. Additionally, many compressors — both hardware and software — will have a signature sound that can be used to inject wonderful coloration and tone into otherwise lifeless tracks. Compression can also be used to subtly massage a track to make it more natural sounding and intelligible without adding distortion, resulting in a song that's more “comfortable” to listen to. Alternately, over-compressing your music can really squeeze the life out of it. For those who are unfamiliar with compressors, having a good grasp of the basics will go a long way toward understanding how compression works, and confidently using it to your advantage.
COMMON COMPRESSOR CONTROLS & PARAMETERS
Depending which compressor you're using, and whether it’s a hardware unit or a plug-in, there are some common parameters and controls that you will be using to dictate the behavior of the compression effect. Below are some of the basic elements of compression. Your compressor may or may not include all of them, but understanding what each one does will allow you to work comfortably with a wide range of compressors.
The threshold control sets the level at which the compression effect is engaged. Only when a level passes above the threshold will it be compressed. If the threshold level is set at say -10 dB, only signal peaks that extend above that level will be compressed. The rest of the time, no compression will be taking place.
The “knee” refers to how the compressor transitions between the non-compressed and compressed states of an audio signal running through it. Typically, compressors will offer one, or in some instances a switchable choice between both, a "soft knee" and a "hard knee” setting. Some compressors will even allow you to control the selection of any position between the two types of knees. As you can see in the diagram, a “soft knee” allows for a smoother and more gradual compression than a “hard knee.”