I've been a blues player for years, but I'm starting to branch out. I recently heard a slap-guitar song and thought, "Shoot! I'll try that!" But within the first few bars I encountered a unique problem that I've never had to work with or care about before.

There's a multi-string hammer-on from open onto the 12th fret with the D, G, and B string, and every time I hit it I hear a lot of dissonance. I broke it down to string by string and realized that every open-string-to-fret hammer-on produces a quiet, but noticeable secondary tone. e.g. hammer on from open B to octave B, C sounds quietly behind it. It's uniform in that for every string, the 12th fret hammer-on has a quiet 13th fret tone behind it, *but* no matter what string, on the 9th fret, the 5th fret tone sounds, on the seventh fret the 6th fret sounds, etc.

This does not happen if my finger is holding down a fret behind the hammer-on; only open-string-to-fret. And before the 5th fret, it only sounds like a natural, metallic tinking sound.

I own an Ibanez AEG5EJP electric-acoustic with a single Venetian cutaway, spruce top, mahogany neck, rosewood fretboard, and a C-shape neck; the action on the strings is about moderate to high.

Like I said it's very a very dissonant sound, and I can't say I've ever really noticed it before on any guitar; however, that could be from lack of paying attention. Is this a problem with many acoustics? Electrics?

The guitar was gently used before I got it, and - other than this problem - sounds fine; also, I've never had any problems like fret-outs, difficulty tuning, etc.

Ideas on how to fix/mitigate...
Last edited by dba14475 at Dec 27, 2012,
I'd suggest putting a capo on the guitar starting at fret 1, and follow the 12 fret harmonic up the neck, one fret at a time. That should give you an idea about how well, or how poorly, the intonation is tracking.

If it's a little off, perhaps you could grind yourself a compensated saddle to improve intonation a bit.

Second, the action should be LAPWOB, as the variance between string gauge and pitch change when fretting, is most aggravated when strings are high.

(This is really annoying with a 12 string, because of the huge gauge difference between the prime and the octave strings).

Intonation error is something that is inherent in acoustic guitars, because of the fixed bridges.
I have already tested the intonation, and know that the harmonics are pretty far off. I think before I jump to conclusions, I'll take your advice and lower the action. That's what's been suggested to me know by a few people (including yourself). Then I'll start looking into getting a new saddle, re-seating the bridge, etc. Work big to small...