Soundboards/stress

[Delwin D Fandrich]

> 
> Bob Hohf wrote
>
> 2.  Dry: bottom of rib - compression, 
>     top of rib - tension,
>     bottom of panel - compression, 
>     top of panel - tension


Hi Bob, Ron, et al,

The best way to measure tension in any structural body is probably with
the use of a strain gage. However, short of that some assumptions can be
made. At least where piano soundboards are concerned.

Ron, that was an impressive set of experiments you conducted. Having
done similar projects, I can appreciate the time and effort that went
into them. However, I must question at least one of your conclusions.

The soundboard panel that was oven dry when it was glued up to your rib
set was entirely under compression as it reached its equilibrium
moisture content (EMC) and stabilized in your ambient climate—whatever
that may have been. In theory, the top of the panel was under slightly
less compression than the bottom of the panel, but it was under
compression all the same. The entire panel was trying to expand as the
wood fibers it is made of swelled up as they absorbed moisture from the
surrounding air. As wood fibers take on moisture they expand, mostly in
diameter, very little in length. All of the wood fibers in any given
piece of wood, no matter where they are located, will eventually swell
by about the same amount unless they are constrained by some outside
force. In this case the panel was constrained by the ribs. In an actual
piano, the rim and the downforce from the string set will also act to
constrain the soundboard panel. Had the ribs not been glued to the
soundboard panel, the panel would have simply expanded and there would
have been no compression once the panel reached its EMC. In any case,
the top surface of the panel cannot be considered to be a separate part
of the panel as if it were a non-hygroscopic film that was simply bonded
to the rest of the panel.

The rib is another matter. The top of the rib is indeed under tension as
you indicate. This is because wood expands and contracts relatively
little due to changes in moisture content in its longitudinal plane. If
you had put the ribs by themselves in the oven and measured their crown
radius both oven dry and later as they reached EMC in their ambient
environment you would have found it little changed. The expanding wood
panel actually tried to stretch the ribs out longer but since wood is
quite strong in tension they resisted that force and were forced into
more of a curve instead. So the expansion of the soundboard panel—due to
the increasing internal compression—forced a change in the radius of the
rib by stretching the top and probably compressing the bottom. The wood
fiber in the soundboard panel compressed because its expansion was
restrained by being glued to the rib. As the radius decreases—i.e., the
crown increases—the bottom of the rib compresses and the top stretches.

I hope all of this makes sense. 


Bob, actually I think the whole practice of crowning soundboards came
into practice accidentally. Until the mid-1900’s ribs were glued up to
soundboards using animal hide glue. To prevent the glue from gelling too
quickly it would be heated up prior to spreading the glue onto the rib
surface. The gluing operation was typically done in a hot room, often a
very hot room. No matter what the moisture content of the soundboard
panel was when it went into the gluing room, it was quite dry by the
time the ribs were glued on. When the assembly was removed from the
gluing room it started taking on moisture. As it did so, it warped.
Ultimately it was discovered that the warped soundboard panels sounded
better than those that didn’t warp. 

Of course, all of this is partly conjecture, but partly it was put
together by talking to some of the old-timers I’ve met who were working
in factories I’ve been to.


ddf