If you are not sure whether it’s a provocative title or a genuinely inquiring one, you’re right. It’s both. If you are serious about audio, mixed some music already, you might already developed some kind of routine. What I mean is, you probably have your moves, i.e. where and what frequency to reach for when you equalize. While it’s a good thing to learn this and be able to work quickly, you might have missed some interesting information about it. Because chances are high that you don’t even EQ the actual musical notes (or more correctly, tones) directly. But what do you EQ then?
Well, either fundamentals, or harmonics (or both). Think about it: an 88 key piano goes from 27.5 Hz to 4186.01 Hz. A 20 fret acoustic guitar tuned to standard pitch goes from 82.407 Hz to 1,046.50 Hz. When you adjust anything above these ranges, whether you cut or boost it, you’re only affecting harmonics. And usually, when you play lower than the top range of the piano or the guitar, you tweak the timbre of the instrument, without changing the balance of the fundamentals… though actually, that’s not entirely true. Because the way comb filtering works, through constructive and destructive interference you can actually affect lower frequencies with changing the level of higher ones. As long as they are sounded simultaneously, of course.
So even when you adjust something lower than the top notes of an instrument, you are really adjusting both some fundamentals, and the harmonics of some of those fundamentals. Why is that important? In itself it is not all that important, as long as you know what to change, and also what to expect from an EQ move. But, if you turn up that 100 Hz on a bass guitar track expecting to get more warmth out of it, and ending up making certain notes of the bass line louder despite the compression, you know you’re in trouble. On the other hand, if you boost that 2 kHz on the same bass track, you get your definition and attack without seriously hurting the balance of the fundamentals themselves. Playing around with harmonics and timbre is a big part of psycho-acoustics. Or you can just say manipulation is a big part of mixing. And once you’ve run out of EQ tricks, you can take out the saturator/distortion device. You can only boost what’s already there to begin with, and even then you might end up boosting noise a bit too much with it. But if you add distortion to sounds, you are creating harmonics; frequencies that were not present previously. That’s pretty much what exciters do. So yeah, harmonics are your friends.