Abnormal resonances

[Me&MyGuitar]

Need advice.
I got a brand new acoustic crossover guitar. When fretted in the middle of the fret, almost all the strings but mostly 4th and 5th sound like if it were two guitars sounding, being one of them just slightly flat. When fretted just near the fret bar, notes come off perfectly clear.
Frets were checked and recrowned to make the top sharper; the fretboard is as straight and flat as a billiard table; the sadlle is made of bone and it was checked, it's ok; there's no truss rod.
Any ideas?

[Alan Carruth]

It's possible that the strings roll on the frets when your finger is back away from the wire. When you pluck a string it is normally moving at some angle to the top; neither perfectly 'horizontal' (parallel to the plane of the top) nor perfectly 'vertical' (perpendicular to the top plane). There is thus some string motion, and force, on the end points at the fret and saddle top in both directions. The horizontal string force can cause it to roll across the fret top a bit. If the string rolls at either end it is, in effect, a little bit longer in the 'horizontal' vibration than it is in the 'vertical' one, and thus it makes two notes at the same time with slightly different pitches. Usually these are close enough together that you don't hear them as different pitches, but do hear a
'beat frequency'. If this is fast enough it can sound like a fret buzz.

The solution to this problem is simple in concept, but can be hard in practice: learn to put your fingers close to the frets... Maybe taller frets would help, at the likely cost of more intonation issues.

[Me&MyGuitar]

Originally Posted by Alan Carruth

....
The solution to this problem is simple in concept, but can be hard in practice: learn to put your fingers close to the frets... Maybe taller frets would help, at the likely cost of more intonation issues.

OMG !!
Well this is my first nylon stringed guitar, but never had such a problem before. Being a crossover the frets are not as wide as on a classical guitar, so putting all fingers close to the frets is almost impossible. At the moment the guitar is stringed with a hgih tension titanium nylon string set; I'm asking whether a lower tension set would help...
And, uh, could it be a "wolf tone" issue?

[Alan Carruth]

I guess it depends on how you define a 'wolf tone'. I usually say that it's a problem caused by one or more resonances of the body or air of the guitar, but there are a couple of 'wolfish' problems that can start out with the strings. I would not call this one a wolf' exactly.

Again, you stated the solution to the problem in your first post: it doesn't show up when you play close to the frets. That was the clue to the nature of the issue, and 'all' you need to do to solve it is to play that way all the time. Easier said than done, of course...

[Me&MyGuitar]

Update.
Putting on the fretboard surface a taller fret to which the tang was filed off: no way, actually the "doubled" note was more evident, all along the fretboard, fretting any of the strings. Trying with taller and taller frets, the resonance becomes more and more evident! It is quite clear that the secondary flat tone comes from the "secondary" diapason between saddle and the point where the finger press the string; this parasite tone can be heard at the attack and peak, while it fades away at the beginning of decay so in the decay phase you can hear only the main tone.
Any other ideas? Any little help is really really welcome.

[Alan Carruth]

"It is quite clear that the secondary flat tone comes from the "secondary" diapason between saddle and the point where the finger press the string..."

So if I understand you correctly, you're saying that the string has two vibrating lengths; one from the saddle to the fret, and another from the saddle to the finger. If the string is breaking over the fret when you press it down, the 'fret diapason' would be the vertical polarization of the string motion, and the longer 'finger diapason' would be the horizontal. In other words, the string is rolling across the top of the fret....

What puzzles me is that the problem gets worse with higher frets. Greater down pressure on the fret top should reduce horizontal rolling of the string.

[Me&MyGuitar]

[QUOTE=.... Greater down pressure on the fret top should reduce horizontal rolling of the string.[/QUOTE]

Nope. It has no effect.

[mc1]

my totally unhelpful contribution to this thread is that, "Abnormal Resonances" would be a good name for an album.

[Alan Carruth]

I wrote:
" Greater down pressure on the fret top should reduce horizontal rolling of the string."
and you replied:

"Nope. It has no effect."

...but the problem goes away when you press the string down at the fret?

[Me&MyGuitar]

Originally Posted by Alan Carruth

...but the problem goes away when you press the string down at the fret?

No, it becomes more evident (does it seem a paradox)

[D. Churchland]

Try this. Take all the strings off except for one of the offending strings. Tune it to pitch and play, see if you get your mystery noise then.

[Ned Milburn]

Originally Posted by Me&MyGuitar

Update.
Putting on the fretboard surface a taller fret to which the tang was filed off: no way, actually the "doubled" note was more evident, all along the fretboard, fretting any of the strings. Trying with taller and taller frets, the resonance becomes more and more evident! It is quite clear that the secondary flat tone comes from the "secondary" diapason between saddle and the point where the finger press the string; this parasite tone can be heard at the attack and peak, while it fades away at the beginning of decay so in the decay phase you can hear only the main tone.
Any other ideas? Any little help is really really welcome.

This is a bizarre way to test a guitar. With the fret tang filed off, the "held on" fret will perhaps vibrate against the fingerboard... So, let's just avoid going further down this testing avenue and return to your original trouble.

You get an unwanted note, but not when fretting close to the fret.

Okay, so proper technique is such that we MUST fret close to the fret without avoiding "phlomp" sounding notes. So, can I be sure that technique is NOT an issue based on what you shared in your writing...?? No I can't. But can I be sure that technique IS the issue?? No I can't.

Another possible issue is back-buzz which is much more prominent on lower sounding strings of nylon guitars. If the nut slots are too low, and if the neck is too flat, the unplayed portion of the string can vibrate gently and cause a pitch or a sound similar to a snare from a snare drum. Try a little piece of business card in the nut slot, then play. If this cures your troubles, then it is likely back buzz. If this were the case, you could get someone to shim your nut slightly & touch up the slots as necessary, or replace the nut entirely and have the artisan leave the slots a bit higher than present.

[Me&MyGuitar]

Originally Posted by Ned Milburn

This is a bizarre way to test a guitar.....

Originally Posted by Ned Milburn

...With the fret tang filed off, the "held on" fret will perhaps vibrate against the fingerboard...

I understand what you mean, but that was a simply way to test whether a too low fret height could be the issue; the resulting sound was exactly the same, only louder the higher was the "fake-fret" we used

As to proper playing technique. Say you're fingering an E chord on the first 2 frets: you will have the tip of your mid finger on the 5th string about in the middle of the 2nd fret, between the two fret bars, and that string will sound with the "parasite" tone; while the tip of your annular will be on the 4th string just close to the fret bar, and it will generate a clear tone. This happens on all the frets all along the fretboard. The "parasite tone" can be heard more or less louder: more on D and A strings, much less on the E-B-G strings but still audible, almost not audible on 6th string.

Back buzz and next-fret buzz have obviously been checked.

[D. Churchland]

This is going to be impossible to diagnose without a video or a sound clip. Can you record a short video showing the problem?

[D. Churchland]

Also, check and make sure that the strings at the headstock are not touching the wood between the cutouts for the tuners