evaluating sdbd. crown & bridge downbearings in a new piano

[David M. Porritt]

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When a dried, constrained SB panel takes on moisture, it is being inflated
like a tire.  It's all under compression.  

dave
*********** REPLY SEPARATOR ***********

On 9/28/99 at 5:04 PM Brian Henselman wrote:
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.  
 
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, 
 
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?
 
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.
 
Frank Weston

Good grief, man, the top piece did not "stretch," you wrapped it around a
larger circle! 
 
_____________________________
David M. Porritt
dporritt@swbell.net
Meadows School of the Arts
Southern Methodist University
Dallas, TX 75275
_____________________________


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