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Hallo folkes,
a friend of mine has bought a very expensive classical guitar from an german luthier. Now testing the guitar with several classic peaces he recognized a "dead" note, I mean a note that has nearly no sustain compared to the others. It is the f# on the treble e at 14th fret.
I think that there are two resonances that eliminates each other. Is that true?
Is it possible to make this note sound as good as the others by changing the thickness of a brace or something like that?

its hard to say without a hands on inspection I suspect a fret or a flat spot in the saddle.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Change the string!

_________________
Mike Collins

In that position, the first thing I would suspect is a loose fret end. But, beyond changing the string as Mike suggested, take it back to the luthier and let him/her solve it. Why waste such a valuable resource?

Are you absolutely sure the fret is all the way seated and well glued in place? You said 14th fret that's over the body and likely neck block making clamping this fret difficult for the builder.

Check this first.

Have you changed strings and brands of strings? I would do this too before messing with resonances.

All good suggestions for mechanical issues to check first. It's a bit of a parlor trick but you can hum into the sound hole and vary the pitch by starting as low as you can manage and then bring the pitch you hum higher. When you hit the pitch of the box you can hear the volume increase and kind of feel the box come alive. If the pitch of your friend's guitar is close to F# this could explain the dead note. It's kind of like the pitch of the box sucks away the corresponding note on the neck.

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Stay with the happy people.
--Reynolds Large

"...f# on the treble e at 14th fret." is three octaves above the 'main air' resonance that you'll hear humming into the hole, so that's probably not the issue.

"I think that there are two resonances that eliminates each other. Is that true?"

It may be close to true. Rather than resonances cancelling each other out the issue is usually the same one that bedevils notes near the 'main air' resonance. Things at resonance are easy to move, so the energy gets extracted from the string and converted into sound very quickly. You end up with a note that is twice as powerful for half as long. Doubling the power doesn't sound twice as loud, so you don't notice that aspect, but the lack of sustain is an issue.

Up in that range there are a lot of resonances in the top, the air, and the back, that can couple strongly with each other. The interaction is non-linear, depends strongly on small details, and is very hard to sort out. In some cases it's possible to visualize what's happening by driving the guitar at the problem pitch and using glitter or sawdust on the top to produce a Chladni pattern. This could reveal an active 'antinode' at the high E area of the bridge. A small amount of added mass someplace on the top or the bridge might shift the pitch or the location of the active area just enough to help. This can also be helped by shaving top braces. Either way it's likely to alter the timbre of the guitar some. Adding mass, by use of poster adhesive ('Blu Tac' or 'Fun Tac) are two brands I've seen) is safe, easy to do, and reversible; shaving braces, not so much, but it's more likely to improve the sound due to the decrease in mass and stiffness.

This is hard enough to diagnose when you have the instrument on the bench, and all the tools for analysis. As always, check the simple stuff first: bad strings (even new ones), all the usual suspects for fret issues (there are a lot..), loose braces, and so on. Good luck.

Bob Gramann Offered some good advice - let the builder sort it out. If you modify anything on the guitar the builder may no longer be inclined to take it back or work on it.