Ukulele Intonation Mystery

[John Tuttle]

I have a problem with a current ukulele build that has me stumped.
I built a concert size ukulele with a 14.75" scale length. Pretty straight forward build.
The problem I have run into concerns the intonation of the third string (C).
Strings 1,2 and 4 intonate perfectly but the 3rd string is another story.
Fretting the 3rd string at the 12th fret does not produce the octave. Fretting at the 11th fret produces the octave.
I have measured the frets for square to the centerline and that is confirmed.
The bridge and saddle are uncompensated. If I were to compensate the C string at the saddle it would be 1/4-3/8" longer than the rest of the strings.

Has anyone seen this before, any thoughts ideas or suggestions would be welcome.

[Clay Schaeffer]

Sometimes uke strings need a bit more compensation than you think they should, but is it possible that when you finger the 11th fret the string presses against the 12th fret and when you finger the 12th fret the string touches the 13th? I am surprised the C string would require a full 1/2 step of compensation

[Peter Wilcox]

If you play a chromatic scale up the fret board on the 3rd string, do the intervals seem to be evenly spaced, or is there a sudden jump as you approach the 11th and 12th frets?

[Chris Reed]

Something is wrong with your build, though I can't work out what from your description.

On a soprano uke, C string total compensation is a little under 3mm (1/8) using Aquila Nylgut strings, and probably under 2.5mm using fluorocarbon strings. (calculated from 2 x nut-12th distance)

For a concert these numbers should be slightly lower - 2.5mm or less as C string compensation.

Forget the 11th for a moment. Does the 12th fret on the C string ring out a whole tone sharp (i.e. as C#)? If so, and if strings 1, 2 and 4 intonate well at the 12th, the most plausible explanation I can think of is that you have a seriously defective C string. If that string has uneven diameter, it will intonate quite randomly. A pretty good test is to swap the string end for end, if you haven't trimmed too much off the free end. If the string is defective, the problem will change markedly (it might sound a B at the 12th, or even be dead on C).

The only other explanation is that your C string is stopped at the rear edge of the nut, rather than the front edge (i.e. the nut slot slopes down towards the fretboard). But that would make the first fret way out, and you'd surely have noticed that and reported it.

Once you have the C intonating properly, I'd guess your 11th will line up too.

[Mark Wybierala]

Peter W... is suspecting the same thing that I am. Do a fret by fret note check starting at the open string. Tune the open string to pitch and with a clip-on tuner verify that each fret properly elevates the pitch by a half step. I believe you have one or more elevated frets.

[Paul Breen]

What kind of strings are you using?

Before you run yourself in circles chasing measurements, try replacing the offending string. I recently installed a new set of Aquila brand Nylgut strings on a 5 string banjo. One of the brand new strings just would not play in tune. Checking the intonation, I found the new string to be drastically off, when comparing the open string to the fretted 12th fret note. I replaced the string and problem fixed. I have used Aquila strings for years and not run into this before but there you go. Plastic strings are NOT immune to intonation problems, be sure to eliminate that first before doing any modifications to your build!

[John Tuttle]

This is GREAT Feedback! I will explore each of these things and report back. Can't thank you all enough for helping me out with this.

[Chris Reed]

What kind of strings are you using?

Before you run yourself in circles chasing measurements, try replacing the offending string. I recently installed a new set of Aquila brand Nylgut strings on a 5 string banjo. One of the brand new strings just would not play in tune. Checking the intonation, I found the new string to be drastically off, when comparing the open string to the fretted 12th fret note. I replaced the string and problem fixed. I have used Aquila strings for years and not run into this before but there you go. Plastic strings are NOT immune to intonation problems, be sure to eliminate that first before doing any modifications to your build!

I used to get the problem quite a lot using nylon strings, and it was usually the thickest (C string). It would stretch unevenly, and you could actually see that it was thinner in some places than others!

I've never had this with Aquila Nylguts, but I've heard of it occasionally. I suspect the cause is that at some point the string was caught by a piece of machinery and squashed in one place. That will be thinner, and so the string will stretch more at that place.

Steel strings tend to announce this kind of damage by snapping, but polymer strings just gaily stretch away ruining your intonation!

[John Tuttle]

Hurray for the MIMF!

A quick update. It was indeed the string itself which was the cause of my intonation woes.
I was so focused on the scale math that I seriously doubt I would have considered that the problem could be with the string!

Thanks again everyone!

[Paul Breen]

Thought that might be the case. Sometimes we overlook or discount the simple things, glad you got off easy on this one!