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I have just one string that is totally dead on the 12th fret. All the other strings (this is a tenor ukulele) are not dead at the 12th fret at all and ring for a long time. What could possibly be going on? Any ideas?

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Formerly know as Mandodiddle.

If it is one of the outside strings, I suspect a loose fret. To test, I tap the frets with a small metal object. I use the edge of a 6" stainless steel ruler...the same one I use to find uneven frets. If it goes 'thunk' instead of a 'ping', then you have found it. I tap several good frets to compare the different sounds.

Sent from my SM-G950U1 using Tapatalk

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John

In addition to John’s advice, if you have not already done so, change the offending string. Bad strings (even new out of the packet) do exist and manifest in strange ways including being dead or making rogue buzz only at certain frequencies.

Start with the string. It's the cheapest thing on the instrument.

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"Act your age, not your shoe size" - Prince

And please let us know what you found the string, the fret, etc? Thanks.

OK, I think I know what my problem is with the partially dead 12th fret. What I still don't know is how to fix it. The problem has been on the 12th fret of my C string on my tenor ukulele.
Everything rings nicely on the C string until that 12th fret. Well guess what happens when I tune the C string down a half step to B. Now it's the 13th fret that is dead. So the uke is somehow dampening the octave C (I don't notice the open C being dampened). The body resonance frequency is just a few cents higher than G#, so I don't think that is the problem.
Any thoughts?

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Formerly know as Mandodiddle.

You may have an undesirable interaction between the note and one of the other resonant modes of the instrument.

Have you tried just experimenting with adding mass to, say the bridge or to soundboard or the sides. Maybe messing around with different size blobs of poster putty.

Other possibilities are difficult or destructive eg brace shaving, changing sound hole diameter.

Trevor Gore posts here occasionally and he is an authority on the topic - perhaps he will stop by and correct my probably addled advice.

Blocking off parts or all of the soundhole will lower the main air resonant frequency. That might tell you something.

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formerly known around here as burbank
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http://www.patfosterguitars.com

The fact that the "deadness" moves with the tuning strongly suggests an instrument resonance right on C. So you know the frequency. Using Chladni patterns, look for a resonant mode at that frequency or maybe an octave of that frequency. When you have found the mode, add a little mass (Blu Tack, probably less than a gram) on an antinode (or two) of the mode. Aim to shift the resonance half a semitone in frequency, then see how it sounds when you pluck the string.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Play C elsewhere. It may not be as prominent, but it should be evident.

Thanks Trevor and everyone. I guess I’ll have to do a little research on how to produce Chladni patterns.

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Formerly know as Mandodiddle.