(Source is: http://homestudiotrainer.webs.com/tipstorecordby.htm)
Its great as a quick guide for beginners looking to get a general idea of how to EQ music. It's also neat because it shows the various frequencies of some onomatopoeia as we understand them by ear. So now you can tell your friends the real difference between a boom and a whack is 100hz and 1khz.
(Everything after this point is just stuff I've learned to do related to EQing your sounds and your mix, not necessarily an ultimate solution.)
As far as I can tell, red bars represent the range within which the instrument's range of sound produces the highest intensity frequency. The highest intensity frequency is the frequency that is loudest/most prominent when the instrument sounds off. Yellow bars seem to represent the range of second highest intensity. Many instruments when producing a sound will have multiple frequencies, some at higher intensities, and some at lower. The combinations of these frequencies represent our given impression of what an instrument should sound like. If you start to cut out those frequencies or add to them, the sound of the instrument drastically changes. (in other words, you recognize a guitar because you've heard its combination of frequencies before. If that combination changes, its no longer exactly a guitar).The varying notes of an instrument of course varies the frequency, but it should all be frequencies within the overall given range suggested above. The piano and pipe organs have rather large ranges of sound and thus produce frequencies almost across the board.
I'm not sure about the black bars, but on the chart, sounds below 20hz and above 20khz are blacked out. Those sounds are inaudible to the human ear. My guess is that black bars represent frequencies that the instrument can produce but have too low of an intensity to either bother with or are practically inaudible when the instrument plays at that range. As such, they should be cut out, in order to avoid overlapping issues which shall be discussed in the next set of paragraphs!
When EQing music, many instruments will have overlapping frequencies at the edges of their range, such as bass overlapping to mid tones, and mid tones overlapping to treble. If this issue is not dealt with, the overlapping edges of the frequencies can often result in a muddy mix. By "muddy" I mean that the two sounds trying to dominate the same frequencies sort of "fight" with each other, and the result can be a dampened or even distorted sound which is unpleasant. If you ever wonder why your sounds seem to become less clear as you add more instruments, this may very well be why.
To deal with this, a good idea is to play the entire mix with all your instruments playing together. While playing, in your equalizer (dedicated to one of the instruments), roll the cutting bands on either side of an instrument's frequency range so that you cut out the low intensity sounds which are being flogged down by more prominent frequencies of other instruments. While it seems at first that this is damaging the instrument by removing some of the frequency, in truth it wont matter if you have other instruments producing frequencies in the overlapping range that are way more intense. Those intenser sounds are outshining the edges of the instrument being EQ'd. That's why removing them when listening to the entire mix together will ensure that you only remove inaudible or unpleasant parts without accidentally removing any sounds that are prominently holding their own. As soon as you hear your EQing begin to change the sound of your instrument, stop, and move back slowly until you hear the sound restored. This should remove the frequencies which can't be heard or are muddying up your mix, and preserve the frequencies which are heard.
(Note: A good example of the above paragraph is cutting any low frequency bass out of your main synth sounds when you have a heavy sub bass. If you leave it uncut, your sub bass can get muddy and even washed down, and your main synth sound can't be heard at those low frequencies anyways, especially if the lower end frequencies are soft and sporadic like they can be in heavily oscillated sounds. Cut those frequencies out to make room for your sub to occupy with a greater intensity. The result is a nice clear sub which fills the range while keeping the prominence of your main synth.)
If you have two instruments in the same range that both need to shine, like for example a sub bass and the sustained sound of a kick, you can control this by side chaining the interspersed or periodic instrument (kick) to the dominant's (sub) volume via compressor or peak controller. This temporarily shuts off the dominant instrument when the periodic instrument sounds, and when the periodic is done, the dominant returns in full force (well, it could actually do a couple things depending on how you've set the attack/release on your compressor, or sustain on your peak controller).
Since many of us electronically produce our own instruments, we can get sounds in far more extreme ranges on that chart above than regular instruments can. You know, like sub bass at 20hz and white noise at 20khz (though at 20khz and passed, you can easily cut those parts of your entire mix without hearing a difference. Same with sounds below 20hz, since that's out of average human hearing range).
I am NOT claiming to be a master on EQing. I just wanted to return something to the community based on the knowledge I've gained, and because the community has already done so much for me. If anybody sees any discrepancies in my words with your own knowledge, please say so, and I'll do my best to update what I said.
