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Dave,=20
I'm impressed, You said in 21 words what it took me 1500 words to say. =
BRAVO!
"When a dried, constrained SB panel takes on moisture, it is being =
inflated like a tire. It's all under compression"
Halleluiah!
I just couldn't have said it any better. Now, much to everyone's =
delight, I'm going to shutup, sit down, and give it a rest. This horse =
has finally died. But it was one heck of a ride.
Now lets all have a group hug and get along!
-Brian Henselman, RPT
-----Original Message-----
From: David M. Porritt <dporritt@post.cis.smu.edu>
To: pianotech@ptg.org <pianotech@ptg.org>
Date: Tuesday, September 28, 1999 6:50 PM
Subject: Re: evaluating sdbd. crown & bridge downbearings in a new =
piano
When a dried, constrained SB panel takes on moisture, it is being =
inflated like a tire. It's all under compression. =20
dave
*********** REPLY SEPARATOR ***********
On 9/28/99 at 5:04 PM Brian Henselman wrote:
Frank,
=20
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.
=20
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
=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.
=20
(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:)
=20
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
_____________________________
David M. Porritt
dporritt@swbell.net
Meadows School of the Arts
Southern Methodist University
Dallas, TX 75275
_____________________________
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