Why does it do this?

SamD

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After 24hrs all the strings on my uke become about a semitone sharper. This seems counterintuitive to me - I would have thought if the pitch was going to change it would shift down?
 
You'd think that, but there are some extra forces at work.

There's a certain amount of friction at the point where the strings come into contact with the slots in the nut. When you tune a string up, you're pulling hard enough from the headstock side that the part of the string south of the nut (fretboard side) is taut enough to get the proper tone. As time goes by, the extra pull from the headstock side overcomes the nut friction and the string gets pulled a bit tighter, raising its pitch.

--Mark
 
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It might also have to do with the temperature - especially if you are using nylon strings. If it's cold the strings will shrink (and get tighter), thus becoming sharper. If it's warm, they go flat.
 
cold will certainly do it - i leave my instruments in the spare room in the house and keep the central heating off or low to avoid damage - first thing in the morning, especially if I have been playing and tuning the night before in a warm living room, I can find the strings sharp.
 
ah yes, I guess it's probably temp dependant. I didn't think it would affect plastic to an audible level, but I suppose it does :)
 
You'd think that, but there are some extra forces at work.

There's a certain amount of friction at the point where the strings come into contact with the slots in the nut. When you tune a string up, you're pulling hard enough from the headstock side that the part of the string south of the nut (fretboard side) is taut enough to get the proper tone. As time goes by, the extra pull from the headstock side overcomes the nut friction and the string gets pulled a bit tighter, raising its pitch.

--Mark

The strings may be binding at the nut, but they are not supposed to.
 
After 24hrs all the strings on my uke become about a semitone sharper. This seems counterintuitive to me - I would have thought if the pitch was going to change it would shift down?

Is it a solid wood instrument?

When humidity goes up in my home, my instruments go sharp.
When humidity goes down they go flat.
 
Yes, humidily will make a solid-wood instrument go sharp, since it increases slightly in size as it absorbs moisture. When it gets drier, the strings go flat. At this time of year, instruments tend to do that! :)
 
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