EQ has two basic rules. Flatter the better. Cut don’t boost. But then you need to know about the effects both good and bad of each frequency, how it may introduce phase interaction and finally what an equal loudness contour graph is.
http://www.idmforums.com/showthread.php?t=18237 for a guide to each frequency and where you use it.
http://en.wikipedia.org/wiki/Equal-loudness_contour for an rough explanation about equal loudness contour.
I found a great explaination of how eq phase interacts…
The way an equalizer works is that you have filters [fixed or variable] that carve out a little piece of the audio… that little piece of the audio is then added to the original signal in either positive or negative polarity [“boost” being the addition in positive polariy, cut being the addition in negative polarity].
Anytime you run any signal through an analog filter you’re going to get a phase shift. The phase shift is a timing issue…
excerpt from an answer http://www.gearslutz.com/board/so-much-gear-so-little-time/29748-analog-eq-phase-issues.html
