Compression can 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. 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.
Alternately, over-compressing your music can really squeeze the life out of it. 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.”
A “soft knee” allows for a smoother and more gradual compression than a “hard knee.”
This refers to the time it takes for the signal to become fully compressed after exceeding the threshold level. Faster attack times are usually between 20 and 800 us (microseconds) depending on the type and brand of unit, while slower times generally range from 10 to 100 ms (milliseconds). Some compressors express this as slopes in dB per second rather than in time. Fast attack times may create distortion by modifying inherently slow-moving low frequency waveforms (Ex. If a cycle at 100 Hz lasts 10 ms, then a 1 ms attack time will have time to alter the waveform, which will generate distortion.)
This is literally the opposite of attack time. More specifically, it is the time it takes for the signal to go from the compressed — or attenuated — state back to the original non-compressed signal. Release times will be considerably longer than attack times, generally ranging anywhere from 40-60 ms to 2-5 seconds, depending on which unit you’re working with. These can also sometimes be referenced as slopes in dB per second instead of times.
Normal compressor operation will be to set the release time to be as short as possible without producing a "pumping" effect, which is caused by cyclic activation and deactivation of compression. For example, if the release time is set too short and the compressor is cycling between active and non-active, your dominant signal — usually the bass guitar and bass drum — will also modulate your noise floor, resulting in a distinct "breathing" effect.
Set your Attack and Release controls to tailor the compression for your source and track.