If you’re reading this, you probably don’t fully understand compressors, and you need to know that it’s ok; most people don’t!
To work with compressors, all you need is a basic understanding of what they do, but if you really want to know what happens under the hood, keep reading!
To understand knee a little bit better, we’re going to go over what compressors do and how they function as a baseline, and then dive into the specifics.
A few different parameters are going to be referenced at some point, so it’ll be helpful to understand them going in.
Compressors receive an audio signal, and when the volume of that signal is over a certain point, they turn it down.
You may have heard analogies for compression, like where a compressor is your mom when you’re playing loud music in your room.
When you play it too loud past a certain point, your mom comes in and tells you to turn it down.
We’re going to stick with this analogy to aid us later on, so bookmark it in your brain.
So the effect of compression is that it takes loud signals and turns them down, while quieter signals are unaffected.
The result is that the dynamic range of an audio signal becomes compressed, hence the name. Pretty simple so far!
The confusing parts come from the rest of the controls that compressors have, which specify when, how quickly, and how much the signal gets compressed.
Maybe the most important parameter to understand, the threshold sets the limit on how loud a sound can get before activating the compressor.
The threshold is measured in dB, meaning setting it at -3 dB will compress only the loudest signals, whereas setting it at -45 dB will activate the compressor on almost any audible sound.
The key is to adjust the threshold to be under the volume of the peaks you want to compress, but still above the parts you don’t want to compress. This requires some tinkering, and depends on the specific audio you want to smoosh.
The ratio determines how aggressively the audio is compressed once it’s activated.
A lower ratio of 2:1 is often referred to as gentle, as it doesn’t do much to affect the volume of the signal. A ratio of 20:1, on the other hand, is close to limiting, where the signal is virtually unable to pass the threshold at all. The effects of this can be quite obvious, and pretty unnatural-sounding.
As a side note, one extraordinarily common misconception with ratios is saying something along the lines of “for every 1 dB of gain past the threshold, the volume is reduced by …”, but that’s not what the ratio means.
Start off with a signal that sustains at -6 db, and imagine the threshold is set at -14 dB. The difference between the threshold and the signal level is 8 dB; this is the dynamic range.
The ratio tells us what the new dynamic range becomes after compression, so a 2:1 ratio of 8 dB of range gives us a new range of 4 dB.
If you want to find the output volume in dB, just add back the threshold: 4 dB + (-14 dB) = -10 dB, which is the new volume of our sustained signal, compressed from -6 dB.
So, what is the knee?
The knee determines how sharply the compressor switches from uncompressed signal to compressed signal. In effect, the knee controls how quickly the ration is applied to signal that fall just above the threshold setting.
Based on this handy graph from Universal Audio’s website, you can see that a hard knee and a soft knee both provide the same output when the signal is a good deal louder than the threshold; the difference is what happens to those signals that are barely above or below the threshold.
To achieve a natural-sounding compression, the goal is to make it so the things we perceive as loud, like yelling, shouting, and explosions, don’t suddenly have their gain reduced in comparison to the sounds around them.
One of our favorite examples of compression that sounds natural but is, in fact, very obvious, is in Moonlight by Ariana Grande at 0:48 where she transitions from a belt to basically a whisper, but they’re both equally audible.
Adjusting the knee can help make the transition between compressed and uncompressed signals smoother, by providing a little wiggle room for the compressor to activate and not be using super high ratios right when it kicks in. As a result, compression with soft knees can sound more organic.
Sometimes it might be desirable to use compression with a hard knee though, like if you want to smash a drum bus through a compressor.
If you’re not going for subtle and just want the gritty, overloaded pumping sound, for example, you might just want a hard knee so that when it’s on, it’s on.
There are other factors that play into the smoothness and naturalness of how your compressor sounds, but the knee works in tandem with them to provide a rigid signal compression or a gentle one.
Hopefully with a better understanding of what the knee does, you can have more precise control over the sound of your compressors and elevate your mixing game!