Soundboard stiffening

[Delwin D Fandrich]

> -----Original Message-----
> From: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]On
> Behalf Of Phillip Ford
> Sent: February 01, 2004 10:16 PM
> To: Pianotech
> Subject: RE: Soundboard stiffening
>
> 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 a spring's
> spring rate is 100 lb/in. then it takes 100 lbs to deflect it the
> first inch.  It takes 100 additional lbs to deflect it another inch.
> It doesn't matter if you start from a zero deflection point or an
> initial 1 inch deflection point - the spring rate (or stiffness) is
> still 100 lbs/in.  It's not getting stiffer because you're putting
> load on it or deflecting it.  True, it takes twice as much load to
> get twice as much deflection, but that is still a constant stiffness.
> I would expect a soundboard to work the same way.  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.

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