plate reaction was Re: Pitch Raising to A440.......Or Not?

[Ron Nossaman]

>
> I don't think this would show that the plate moved. I have done this, in a 
> sense. I have removed the bass strings, and then listened to the treble. Yes,
>
> they were low closer to the break. But I would think that is because there is
>
> less tension on the board towards the back, where the bass bridge is. Not 
> because the plate rose.   


Hi Wim,
It's not exactly that the plate rises, though it may. It's that it bends
between struts in the direction the strings are pulling it, shortening the
strings and dropping the tension of those already tuned. For some time, off and
on, I've been playing with some spread sheet simulations to see how much you
have to stretch and relax a string to change pitch a certain amount. It depends
on overall length, lengths fore and aft of the bridge, wrap, and tension as
percentage of breaking strength as to how much movement causes what pitch
changes. On an A-440 of 410mm speaking length (more or less ignoring the other
lengths for now, though they would have an effect), changing the downbearing
angle from 0.5° to 0° would shorten the speaking length about 0.00001", and
lower the pitch about 0.05 Hz. That's forcing the board down from a 0.5°
bearing to 0°, which isn't likely to happen in the worst case. Figuring in the
back scale would amplify the change, and adding the rest of the lengths in
would diminish it again. Even if the pitch change with all the other string
lengths factored in was ten times the figure of just the speaking length, it
still wouldn't be enough by far to account for the pitch drops we regularly see
during pitch raises. The plate foreshortens along the string plane as tension
is added, and a little plate flex makes a lot more difference in string pitch
than a little soundboard deflection. Really, it does.


Ron N