Physics and Guitar Intonation

[Etienne Rambert]

Please correct me if I'm wrong. I'm sure many of you know more about this than me. Here are my assumptions:

 

1. The tendency of a string that is tightened to pitch would be to loosen over time, unless there is another physical force causing the tension to increase.

 

2. As a string loosens, it will fall below pitch. It goes flat.

 

My question - What physical forces would commonly cause a guitar string's intonation to move toward the sharp end of the scale? I've noticed that quite often, my guitar strings will not loosen and go flat. Rather, they will move a few cents sharp. At other times, they do what I think is the normal thing, they move a few cents flat.

 

This is true on all of the steel-string guitars I've owned, regardless of where & how they were made.

[Fantfool]

Temperature changes can cause the strings to go sharp due to the wood moving and tightening the string tension.

[LaurentB]

Indeed, when it suddenly becomes colder the strings go sharp. Maybe because the strings shrink more than the would when becoming colder.

But I've noticed also strings to go sharp when I've tuned them down after them having been at a higher pitch for a long time. When you drop the low E to D for instance.

While plastics can have a kind of 'memory', wanting to return to the old shape, metals don't have that, so I wonder how to explain that. Would it be the neck resetting itself a bit?

[Krash]

Yep. Temp/humidity changes can affect tuning in both directions. The strings on my semi-hollow electric wiil go all sharp or all flat, by approx. the same amount, with changes in the weather. It affects the wood, not the strings.

[drnihili]

 

While plastics can have a kind of 'memory', wanting to return to the old shape, metals don't have that, so I wonder how to explain that. Would it be the neck resetting itself a bit?

 

 

Metals do have memory. That's why new strings spring out straight when you take them out of the package and undo them. You can overcome the memory with sufficient deformation, but they do have it, and it's essential for producing the proper tension for intonation.

[LaurentB]

 

Metals do have memory. That's why new strings spring out straight when you take them out of the package and undo them. You can overcome the memory with sufficient deformation, but they do have it, and it's essential for producing the proper tension for intonation.

 

 

No, that's not what I mean, that's simply elastic deformation, like that of a spring. Material memory is plastic deformation (i.e. lasting deformation, also when the stress is relieved) which however creeps back over time a bit towards its original shape.