Soundboards...a little closer to reality

[Ron Nossaman]

>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'???

* All this would accomplish is to further damage the panel. In that regard,
I suppose some of the internal stress would, indeed, have been relieved, but
to no benefit. 


>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?)

* Chris Robinson compression crowns boards with straight ribs in a dished
press @ 5% EMC. To set bearing in the piano, he temporarily forces the
crowned assembly flat in the piano to establish the bridge height so that
zero bearing corresponds to zero crown. The theory is that, as the panel
ages, and the crown diminishes, the bearing does too, so it's not possible
for the string load to force the soundboard entirely flat. Not a bad theory,
in my opinion, and I set up my RC boards so there is slightly more crown
than there is bearing for the same reason. What I don't do is force a board
flat to set bridge height. That can be done by measurement without
potentially traumatizing the board.  


>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.)   

* Yes, people really do put that much downbearing on them. I have found over
2mm negative crown in the killer octave of new pianos with two degrees of
bearing in the same area. If the soundboard had any crown at all when the
bearing was established, it had to have been set at around three degrees. I
don't usually find Ds with excessive bearing, except in the extreme treble,
where it seems to run at about 2 1/2 degrees. Some folks may not think
that's excessive, but it's about a degree high for my taste. The worst crown
and bearing measurements in Ds seem to be in the killer octave, where the
board is flat or concave, and the bearing is nearly zero, or negative.
That's not a bearing setting problem though, that's a soundboard design
problem. The bearing was probably pretty good when it was first strung. 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...)

* Exactly the right approach, I'd say.


>
>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.?

* Perhaps, but the 100 year old ones that  are still viable are the
exception, rather than the rule.



>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?

* I wouldn't want to flatten one out, but if it was done, it would do
considerably less damage to a RC board than to a CC. Yes, excessive
downbearing can kill a RC board too. You want enough bearing to deflect the
board enough to raise it's impedance to accommodate that of the string
plane. At the same time, you want enough bearing to get good coupling and
energy transfer between the strings and the bridge, while not having so much
as to restrict the movement of the soundboard or drive the crown flat. It's
rather a slick balancing act, and the RC board can give you much better
control of the process.

>
>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
>

* Just keep putting the pieces together. It all fits somewhere, and it all
makes sense.

 Ron N