Soundboard stiffening

[Phillip Ford]

> > David,
> >
> > This is the point that I was trying to make.  I believe that the
> > soundboard is essentially just a big spring.  A spring doesn't get
> > more rigid or stiff the more you compress it.  Within its working
> > range it has a constant spring rate or stiffness....If the board is
> > actually getting stiffer as a result of applied load then it's not
> > acting like a spring or a beam, and I would like to understand what
> > mechanism is causing that to happen.
> >
> > Phil Ford
> > _______________________________________________
>
>But not all springs are linear. Some are non-linear by design.

That's true.  But if I was trying to design a non-linear spring my first 
choice would not be an essentially flat plate with ribs attached to it.


>I think I started this notion based on some measurements I did at Baldwin. I
>was working with a new Model L that had been bellied but not strung. I
>wanted an idea of how much force it took to take the board from its natural,
>fully-crowned condition down to half the distance between that and flat. The
>exact numbers I don't remember (nor do I have access to them) but weight was
>added in 50# increments and the deflection measured with a dial depth gage
>setup mounted on an aluminum bar clamped to the top of the rim. The depth
>gage was measuring to a point about 1/3 of the way up the tenor bridge. The
>deflection was not at all linear up to the point we stopped adding weight --
>that is, at 1,000 pounds. That took it to not much more than flat.
>
>These boards are (were?) partially rib-crowned and partially
>compression-crowned. And Baldwin boards are (were?) rather lightly crowned.
>I do not know if the same, or similar, results would be obtained with either
>a pure compression-crowned soundboard (I suspect so) or a pure rib-crowned
>soundboard (perhaps not, depending on the rib contours). Nor do I know what
>would have happened if we had continued to add weight beyond this amount --
>we were only able to borrow 20 fifty pound weights.
>
>Del

You say that the deflections were not linear.  Were they incrementally 
decreasing for unit increases in load?

At the risk of starting the RC vs. CC debates again, if a CC board were to 
get stiffer with increasing load, but not an RC board, would you say that's 
a significant difference between the two?  I thought your position was that 
there is essentially no difference in behavior or performance between an RC 
and a CC board.

Phil Ford