These misconceptions regarding compressors are very common
Compressors were around for a while, so it shouldn’t be a problem for audio-interested people to understand its basic concept and features. However, checking around on internet and other media – all kinds of forums, websites, even books written on the subject of mixing, mastering and the likes – makes one realize that many people still get certain parts or even the whole thing wrong. So let’s clarify some of the most common ones of these misconceptions.
I. “Release: How fast the compressor releases the signal after it moves below the threshold.”
Wrong. The release time sets how fast the compressor decreases gain reduction in order to attempt to achieve the set compression ratio.
Understanding this statement will make you realize that releases happen above the threshold all the time, the audio signal level doesn’t have to fall below it for them to happen. If the above quoted, wrong definition were correct, a strong enough signal staying above the threshold would never, ever get released. If you have a great set of ears to be able to hear compressors in action, you know that’s not the case. Certain compression-related effects like the so called “punch”, “click” or “knock” would not be possible if compressors would work the way the above, wrong definition stated.
II. “Attack: How fast the compressor grabs the signal and applies compression once it hits the threshold.”
Well, at first glance, this seems to be correct, right? Because compression, indeed, can only happen once our signal is above the threshold. But the definition above is still wrong, because a compressor doesn’t only attack the signal at moments when it crosses the threshold. Instead – once the signal is above the threshold – there will be an attack phase every time the gain reduction needs to increase in order to attempt to reach the given compression ratio.
You can easily realize that if compressors worked the way the wrong definitions stated above, attack and release times would become kind of irrelevant, because the action of the compressor would depend mainly on when the signal crosses the threshold from either direction. That way, the attack and release times would simply introduce a delay after which the actual action would happen. And this incorrect interpretation of compression leads us to the following misconception.
III. “Attack and release are set time intervals after the actual attack or release actions will happen to the signal.”
Wrong. Attack and release are the amount of time it takes for the compressor to increase or decrease the amount gain reduction, so it can attempt to reach the amount of compression set by the ratio. So once an attack phase happens – practically immediately at each transient – we can set how long it will take for the compressor to gradually increase the gain reduction level to the amount we have determined by setting up the “ratio” knob. And similarly, with the help of the “release” knob we can set how quickly the gain reduction level will fall.
Also, to be precise, the time you can set up with the attack and release knobs are usually not the actual amount of time it takes for the gain reduction to raise or fall. Rather, it’s the amount of time it takes for the gain to change by a predefined amount in dB, which is often 10 dB. The compressor’s time constants are referenced to that dB amount.