Critical Bands – the quickest, most useful article you’ve ever read about equalization
When it comes to hearing pitch (and frequency in general) your ear and your brain have a weird physiological quirk. Low sounds will start to mask higher sounds of equal volume.
Simplified, this means your ear has frequency “bins”. Anything in that bin will blend, with your ear deferring to the most powerful frequency (with a bias towards low frequencies). This has many uses from audio compression algorithms to advanced EQ techniques.
There are charts and whatnot, but here’s a useful shorthand:
- Frequencies below 600Hz will start to mask when they are <= 100Hz apart.
- Frequencies above 600Hz will start to mask when they are <= 1/3 of an octave apart. You can calculate the top bound (f2) of a range starting at f1 like so: f2 = f1 * 2^(1/3)
While it largely depends on the audio material and its frequency content, you can theoretically create very granular bins, or frequency ranges for mixing. Blending and separating become more attainable at a more precise level.
(Keep in mind that these bins aren’t totally concrete; you won’t magically hear something clearly once it’s above that 1/3 octave; as with most things, it’s a gradient.)
How to use with EQ
Say you want your jangly guitars to blend with your piano in the mid range. You notice the guitars have a nice-sounding peak around 1kHz, and you want to use that as the most dominant characteristic in that bin. Give the guitars a bump there. Next, find the top of your bin range.
1000 * 2^(1/3) = about 1260Hz
Add a bump between 1kHz and 1.26kHz in your piano (adjusting the gain based on how energetic it was before the boost).
If you’d wanted your piano to have separation from your guitars, you’d just move that bump outside of the 1–1.26kHz range.
Use your ears. This technique can be a useful starting point, but when something sounds “cheap” or generally undesirable, try something else.
You must also consider the frequency content of an instrument before EQ. If the hypothetical piano already had lots of energy at 1.2kHz, you probably shouldn’t be boosting there. Think of it like putting together puzzle pieces that you can surgically alter to fit better. The bulk of the shape is not something you want to change, because it will be an uphill battle with ugly results.
The goal here is using psychoacoustic principles as a spring-board to creativity. Use the concept of critical bands to simplify your mixing struggle, not to make it more difficult.
Thanks for reading!
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