evaluating sdbd. crown & bridge downbearings in a new piano

[Brian Henselman]

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Frank,

Nothing is PULLING the top to make it expand (hence, there is NO tension =
in either a compression-crowned/straight rib or a tapered =
rib/rib-crowned soundboard).  The expansion is coming from WITHIN the =
wood cells, because as internal water content increases, internal forces =
begin to push outward.

When humidity is added to a dried soundboard, all of the cells, =
including the top, are GROWING from within and pressing outward.  This =
is not tension, but compression.  Why?  The argument here is caused by a =
mis-understanding of term "compression".

Try to bear with me here.  You and I usually think of compression as the =
increase of force on a cell from the OUTSIDE pushing in.  This =
definition as it pertains to wood is fine as long as the water content =
stays constant, and the only variable is the increase of pressure from =
the outside. =20

When internal water content stays constant but outside pressure is =
increased, the cell shrinks from the increase of outside pressure =
pushing in, while simultaneously raising the pressure inside too.  This =
is a very simple example of compression.  One where the cells are =
shrinking.  However, increasing the pressure on the outside is not the =
only way to create compression.

(BTW, when you flex your yardstick, you are "compressing the wood on the =
the bottom" while maintaining a constant moisture content.  You are also =
stretching the outer/top side by placing it under tension.  However, =
this doesn't apply to  manufacturing a soundboard.  Here's why:)

Think of wood cells with a small amount of water in them (i.e. a dried =
soundboard panel).  If we simply added water to the inside by raising =
the surrounding air humidity, the cells would grow.  Would this growth =
alone cause compression? No, not necessarily, because the cells aren't =
being constrained by any additional outside force.  (Del, Ron, Brian T, =
I hope I'm right on this point).

Now constrain this growth, by first drying out the panel, and attaching =
a rib to the bottom of the soundboard panel.  Now reintroduce humidity.  =
This increased humidity cause the internal water content to increase, =
thus increasing internal pressure, which then introduces the compression =
to the system for the first time.  The lower side (ribbed side) is under =
compression because the cells are filling with water, but not growing =
because the rib won't allow growth.  But, what about the top side?

Aha!  The top IS growing, but from the inside---> out!  This is =
COMPRESSION, albeit less than on the bottom side, but still compression =
nonetheless.  "But How? The top side is stretching, right?"  Here's how:

Think of "tension" for a moment.  Tension is caused when an object is =
forced to stretch from a force or forces PULLING from the outside.  When =
you flex your yardstick, you are "pulling" the outside to form the =
upper/outer arch.  Same goes to Steve and his canoe analogy.  These ARE =
examples of tension.  Yes, the top of a crowned soundboard has =
"stretched" because it has grown, but this growth is NOT from outside =
forces PULLING it into this shape, hence there is no tension and there =
never was.

As I stated in my first sentence, NOTHING is pulling the soundboard to =
create the arch.  "What about that rib?"... you might ask.  No ribs, not =
even tapered ribs, PULL the soundboard into an arch.  They function as =
stationary objects that either follow the arch (as in tapered ribs), or =
resist the arch (as in straight ribs).  However, these ribs  neither =
shrink, nor grow to create the arch (ok, it's wood... so they do did =
grow, but only a LITTLE, ok).  However the arch is caused because all =
ribs RESIST growth, and limit growth on the bottom by "holding" it, BUT =
THEY DOT NOT STRETCH the top.  The top is simply expanding from the =
growth associated with the increase in internal water content.

Just because an object "stretched" doesn't mean it is under tension.  =
Cells swell/grow as they fill with moisture, however "tension" pulling =
outward does not cause this growth.  This growth is caused from =
"compression" forces pushing outward from within the cell.

Are the cell membranes stretching?  YES, but this is not TENSION!  Why?  =
Because the cells will continue to grow until the water content inside =
the cell EQUALS the water content outside of the cell.  If you keep =
increasing outside humidity levels, what happens?  Well, throw your =
panel into water, 100% humidity (and for the purpose of this ludicrous =
example let's pretend that your glue hold the ribs isn't water =
soluable).

Do we still have tension in on the top side of the soundboard?  NO.  =
Even though the cells have reached full saturation and have expanded to =
their maximum natural capacity, the rib is still not STRETCHING the top =
"outer" surface.

BTW, unlike Steve's Canoe analogy, when manufacturing a soundboard, NO =
ONE is grabbing the wood and STRETCHING it.  I'll say this for the last =
time:  A soundboard's arch is formed from increasing pressure from =
within each cell, not from an outside force PULLING outward.  Hence =
there is NO TENSION in either a compression-crowned nor a rib-crowned =
soundboard.

I hope this clarifies things into laymens terms.

-Brian Henselman

ps, Please forgive any glaring typos in that I was typing this while =
holding a tantruming 2 year old in my lap.  NOW THAT'S TENSION!!! Heh =
Heh.
  -----Original Message-----
  From: Frank Weston <klavier@annap.infi.net>
  To: pianotech@ptg.org <pianotech@ptg.org>
  Date: Tuesday, September 28, 1999 12:15 PM
  Subject: Re: evaluating sdbd. crown & bridge downbearings in a new =
piano


  Del,=20
  =20
  You are really causing me to think now.  Are you saying that the =
tension in a bent (or wrapped) piece of wood won't cause it to stretch =
on the convex side?
  =20
  This idea seems to be contrary to the basic Law of the Universe that =
states everything has got to be somewhere.  If the top side of a bent =
piece of wood does not stretch and the bottom shorten, careful geometric =
analysis demonstrates that as you bend wood, are changing the amount of =
wood in the cosmos, and we all know that only God can make a tree.
  =20
  Frank Weston

      Good grief, man, the top piece did not "stretch," you wrapped it =
around a larger circle!=20
      =20


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