Soundboard stiffening

[Delwin D Fandrich]

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  -----Original Message-----
  From: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]On
Behalf Of Richard Brekne
  Sent: February 04, 2004 1:46 PM
  To: Pianotech
  Subject: Re: Soundboard stiffening


  Delwin D Fandrich wrote:

Now, wait a minute here. Who said "the ribs are doing nothing but trying to
straighten out the panel."? With or without crown, and regardless of how
crown is obtained, the ribs are going to add stiffness to the assembly. That
does not preclude their resisting being bent in the compression-crowned
soundboard assembly.

  You guys do !  And quite frequently. Ok...in the context of rib supporting
crown. But forgive me... I am a bit foggy on how it is that ribs that in
that context do nothing but resist crown and offer no support against
downbearing, can at the same time not only make the panel stiffer (which
makes it harder to press down.. hmmmm ) and not only that but makes the
system increasingly stiffer (even harder to press down) as more downbearing
is applied.

  I am just sure I am crossing Z's with Ø's here... but these two positions
seem a bit contridictorary to me.

So, let's see. We're not only supposed to figure out how to explain a
principle in words you can't possibly dissect and render other, unintended
meanings to but we're also supposed to figure out how to do this in a way
you can't possibly take out of context and render other, unintended meanings
to. OK. I think I've got it. Of course, you realize that does limit both the
ability and the desire to communicate with you.

No. there are not two positions. There is one unified theory of operation
and it contains no contradictions. The whole system works together.  Think
of it as having more than one thing going on simultaneously.


No one talks about "the panel trying to straighten out the ribs" because it
doesn't work that way. Have you ever handled an edge-glued spruce panel?
Sure you have. Just how much straightening is that thing going to do?

  What I see is two pieces of wood that are forced into a curved condition
against their better judgement... and both... would return to being flat if
they had their ditthers about it.  Why would the rib be pulling on the panel
to straighten it out, and not the other way around in equal amounts?
Oh, think about it! Because the rib, longitudinally, is quite stiff and
resists being bent with some vigor. And the soundboard panel, cross-grain,
is quite flexible and bend readily. So readily, in fact, you have to handle
it gently until those nice, stiff ribs get glued on.


I would suggest that the ribs are not bent simply because the panel is
taking on moisture... but because they are made to  resist any panel
expansion at the interface between ribs and panel.  It seems only half a
truth and one that easily leads to less then half truths to simply say
the ribs are bent by the panel, and do nothing more then resist crown.

I think you're getting the idea. In the compression-crowned soundboard
assembly, the ribs are indeed bent because of the stress interface between
them -- they really don't like to stretch along their grain -- and the
expanding soundboard panel. (At least the panel would be expanding if it
were not constrained by the ribs glued to it.) And the panel is expanding
because it is taking on moisture. That is how a compression-crowned
soundboard assembly works.

  Well heck there Del... this much has been clear all along... and of course
we dont want the ribs to actually strain along the grain... but that doenst
mean they dont feel the increase in stress in that direction. If they
strained or failed... then crown would disapear as well.
Of course the ribs are going to strain along the grain. At least a little
bit. That is a natural effect of the stress interface built up between the
top surface of the rib and the bottom surface of the soundboard panel in
which we've created some level of compression that we have glued together.
As the soundboard panel takes on moisture and tries to expand it forces the
top of the rib to stretch slightly. Simultaneously the bottom of the rib to
compress slightly. This is what happens when you force a bend into an
otherwise straight piece of wood. And being strained in this way it wants
very much to return to its neutral state. So it pulls against the crown.
Continually. That pull is resisted by its being glued to the soundboard
panel. The rib does not have to fail for crown to dissipate, all it takes is
for there to be a loss of compression within the soundboard panel. At which
point the rib returns to its neutral position (which is what it was trying
to do all along) and is no longer strained.

Del

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