EQ build, selecting important freq.?

[donald stringer]

I am about to build an eq section for a project and need some input for selecting the proper values. To simplify the selection process I could use the freq. values that equal the six strings on a guitar from low to high with maybe some inbetween values. Does this make sence ? What may make some good selections that may be outside what are normaly provided for in a typical EQ pedal.

[Seljer]

Well look at some commercial EQ pedals to get an idea of what range they cover. The Boss GE7 has controls for 100hz, 200hz, 400hz, 800hz, 1600hz, 3200hz and 6400hz. That MXR 10 band has got 32hz, 62hz, 125hz, 250hz, 500hz, 1000hz, 2000hz, 4000hz,  8000hz and 16000hz.

You may notice that every one is roughly 2x the amount of the previous one, thats means that each slider controls a frequency one octave higher then the previous one.

Your average guitar has got 82hz for the 6th string and 329hz for the 1st string, and if you move up the fretboard you eventually end up somewhere past 1000hz at the top of it depending on how many frets you have. Bass is an octave lower than a regular guitar, so about 41hz, or 31hz if you're into the 5 string stuff with the low B
However, that is not the important thing, because when you play just one note, it doesn't ring with just that one frequency, you also get a bunch of harmonics (that are all multiples of the fundamental frequency) that cover quiet a lot of range.

heres the spectrogram you get when you pluck the 4th string

(recorded through my amp, on which I just realised i had the mids cranked up)

the content of these harmonics really defines what your guitar sounds like so you want to be able to control them with the EQ pedal.
A typicaly guitar amp EQ covers everything from around 100hz with the bass control, something between 300 and 800hz with the mids control (depends on the voicing of the amp), and everything past that (1khz and up) with the treble control. You also have to remember that we stop hearing stuff past 20khz or so (and below 20hz), and regular guitar speakers are really really poor at reproducing anything past 7-9khz.


Most guitar EQ pedals like the Boss I mentioned and other ones have got sliders for each octave (start from your lowest frequency then multiply x2 for each next one), some larger rack EQs, which have got 15->30 bands have got sliders for each 1/2 octave (multiply x1.414, the square root of 2) or 1/3 octave (multiply x1.26 roughly, the cube root of 2)