Soundboard crown

[Delwin D Fandrich]

----- Original Message -----
From: "Phillip Ford" <fordpiano@earthlink.net>
To: <pianotech@ptg.org>
Sent: August 15, 2003 11:15 PM
Subject: Re: Soundboard crown


>
> I keep reading about this increase in soundboard stiffness and I
> don't understand what it means.  Are you distinguishing between
> preloading and stiffening?  By preloading I mean that if you put more
> load on the board you get an increase in preload.  Put on 100 lbs and
> you have 100 lbs of preload.  Put on 200 lbs and you have 200 lbs of
> preload.  By stiffening I mean that as you deflect the board it takes
> increasing increments of load to achieve the same increment of
> deflection (like a spring with an increasing spring rate as it is
> compressed).  Put on 100 lbs and you get a deflection of x.  Put on
> another 100 lbs and you get an additional deflection less than x.  Is
> this what you mean?  Do you have some soundboard deflection vs. load
> data to substantiate this?

I measured the deflection per given amount of weight on a Baldwin Model L
grand. It had a new factory board--a combination of compression-crowning
and rib-crowning technology--and the piano had not yet been strung. We used
50 pound weights borrowed from a local gym. We placed the weights on the
bridge--wedging them up to keep the stack from tipping over--about in the
middle of the soundboard just a bit below the tenor/treble break. (I don't
have one of these handy or I might be able to be more specific. Memory
fades after 15 years or so, but as I recall the weights were positioned
directly over the longest rib.) We measured the deflection after the
addition of each weight. The initial deflection--measured immediately after
the addition of the first weight was the greatest, decreasing with each
added weight. I don't remember the numbers but I do remember that it took
about 800 pounds to bring the board down to approximately zero crown. And
we stopped at that point. (This was all done during the winter when the lab
was relatively dry. I expect it would have been some higher in the summer
heat and humidity.) There was some obvious distortion developing in the
board, i.e., the region immediately around the bridge was just touching a
string stretched across the bottom of the board from one end of the rib to
the other and there moderate upward bulges between this region and the ends
of the ribs. This effect is common to compression-crowned soundboard
systems and is illustrated in one of my Journal articles.


>
> It seems at odds with your belief that all the downbearing is
> supported by rib bending and none of it by panel loading.  Beams
> don't get stiffer as they deflect.  If it takes 100 lbs to deflect a
> beam 0.1 inch then another 100 lbs will deflect it another 0.1 inch.
> If the soundboard is getting stiffer as it is pushed down then
> something must be reacting that load other than just rib bending.

You do raise some interesting points. But to set the record straight, I do
believe that panel expansion has some effect on even a rib-crowned board.
At least initially. We rib our boards with the panel MC at 6.5%, or as
close to that as we can hold it, With any moisture content over that amount
there is going to be some stress interface between the panel and the ribs,
hence some crowning effect contributed by the moderatly compressed panel. I
just don't count on that for any long-term crown support. All of my rib
calculations are made as if the panel were neutral.


>
> If it's not getting stiffer, if instead its preload is increasing,
> then the same effect could be achieved on a flat board.  And there
> must be another explanation as to why a crowned and preloaded board
> seems to give better performance than other arrangements.

One of these months I'll have to try my experiment again. This time with
one of the boards designed the way I'm designing and building them
currently. The Baldwin boards were (are?) primarily compression-crowned
with a moderate curve machined into the ribs serving more-or-less as a back
-up. Ours are as nearly completely rib-crowned as I can make them. The 6.5%
MC is intended to prevent the panel from being exposed to excessive
compression even in the most humid climates common to the U.S. As well,
they should not become excessively tensioned even in the driest climates.
I'm inclined to accept a bit more compression than tension since with
spruce the compression strength is generally somewhat better than the
tension strength.

Del