Soundboards...a little closer to reality

[Brian Trout]

One of the concepts that I've seen stated in regards to CC (compression
crowned) soundboards is the idea that if you dry down the board, glue on the
ribs, let the board be crowned by the absorption of moisture, and then
flatten it back out again with the pressure of downbearing, that somehow
you've relieved some of the internal stresses of the soundboard / rib set
unit.  (I could have taken it wrong, I've been known to be dense at times.)

I'm trying to figure out if any such thing has any basis in the reality of
quality piano construction at all.  Assuming I did intend to build a
soundboard that was compression crowned, (I never have, so I'm only
speculating), would I really want to even come close to 'flattening it
out'???

Let me address a point that came to mind.   I recently visited a rebuilding
shop, the name of which many of you would know and respect, that was showing
some of their basic procedures.   They do only rib crowned boards.  For a
board with a crown corresponding to a 60' radius, a rib span of aprox. 4'
equals about 3/8" of crown at it's highest point.  (approximately)  The
rebuilder mentioned that under tension, his boards were moving towards flat
by aprox. 3mm in the center.  (Mine usually move a little farther, but we're
not talking about mine at the moment.)

Does anyone really approach actually flattening out their boards?(on
purpose?)

In the real world, wouldn't a soundboard cease to have any bearing at all
before the board went completely flat?  Or do people really put that much
downbearing on them?  (Maybe that might explain a relatively new "D" with a
flat board.)   I don't know, I'm throwing out the question. (And I am aware
that there are a few more factors such as length of backscale and such that
would change the figures.  I was trying to be a little more basic in my
thinking, more of an overall picture...)

In the real world, I could see how a compression crowned soundboard assembly
could actually have negative bearing exclusively because of the collapse of
the crown in the board.  The soundboard assembly is in a constant battle of
forces within itself to collapse. And downbearing only adds to the forces
that work toward flattening it out.  And after a while, (what my eyes have
seen leads me to believe that) they eventually do loose the battle and
flatten out.  Some are 100 years old, some are only 4 or 5 years old.  I've
seen them with my own eyes.  Perhaps Brian H's comments about the quality of
spruce available would be particularly relevant here.?

I don't see the same concepts in rib crowned boards.  The natural state of a
rib crowned board is crowned, not flat.  The internal stresses increase as
the board is forced toward a flat state, which is what happens when
downbearing is applied.  But here again, does anyone who's building and
installing these things really want to flatten this thing out?  Would that
be a possible reason for early failure, a much heavier downbearing than is
necessary? (or desirable?) In either compression crowned or rib crowned
boards?

Just thinking out loud, with my mind wandering around a bit.  I don't have
the answers, heck, I still have trouble asking the right questions.! :-)

But thanks to all of you who have contributed to the discussions we've been
having.  It's interesting, and perhaps we'll all learn a bit.  :-)

Wishing you all a fine and good weekend,

Brian Trout
Quarryville, PA
btrout@desupernet.net