Most fully parametric EQs have only one to three adjustable frequency bands, but for each band, you can control how much the band is boosted or cut and also change the center frequency of the band. In addition, you can control the “Q,” or the width of the band. Since all three controls—cut/boost, frequency, and Q—interact, adjusting them takes patience.
Always start with the controls flat. One easy way to get started with a parametric EQ is to simulate a simple three-band tone control. For example, to simulate the effect of the tone controls on a typical combo amp, set one band’s frequency control to 60 Hz, the next to 400 Hz, and the third control to 13 kHz. Set all of the “Q” controls to their broadest setting.
The primary benefit of a parametric EQ, however, comes when you find that these starting frequencies aren’t quite what you need. Maybe you are using alternate tunings or a baritone guitar and need the bass control to be centered at a lower frequency. Just change the first band of controls to a lower frequency. Or perhaps your guitar seems a bit muddy, and neither cutting the bass at 60 Hz nor the midrange at 400 Hz fixes the problem. Try cutting the second band, which you’re using as a midrange control, and then vary the frequency control slightly above and below 400 Hz to find the best sound.
Excerpted from Equalization and Effects