Soundboards/stress

[Ron Nossaman]

Hi Del,

Wrong guy. Bob made the models, not me. I'm the guy that's been arguing the
very point you just repeated below. Thanks.

Ron Nossaman




>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. 
>
>ddf


 Ron Nossaman