Soundboards/stress

[Ron Nossaman]

Hi Bob & all,

Thanks for the response. A question or two, if you will, and comments.



>To determine what was under tension and compression, I made "gauges" out of
the 
>extra ribs and the waste of the soundboard panel.  A "gauge" rib could be held 
>against the concave sides:  if the soundboard curvature was greater than the 
>gauge, the concave side of the panel was under compression; less than the 
>gauge, 
>tension.  To check the convex side, I took a strip of the panel waste,
lined it 
>up with the edge of the "soundboard", and "rolled" it over the surface to 
>compare the dimension.  If the dimension of the "soundboard" was greater than 
>the length of the waste strip, I took that to mean tension.  Since the gauges 
>and the soundboards were all at equilibrium with the ambient humidity, the 
>dimensional changes due to humidity should be equal.
>


Hmmm, interesting. How much longer than the "control" was the convex side of
this board. How much shorter was the concave? Did you compare the contacting
surfaces (concave of "control" to convex of board) or the convex surfaces of
each for length (squared up from a flat surface) when measuring? What was
your crown radius? I would think that anything significantly smaller than 60
feet would skew the results toward what you were looking for, as would the
shallow rib since it would deflect more and decrease the radius more. Sorry,
I'm still not convinced that the convex side of a (new) soundboard is under
tension (especially under string load) because the dimensional change
between dry-down for assembly, and swelling with rehydration is much greater
than is accounted for by the resulting crown (from my previous post). 

Good point about crown being primarily structural. I'd imagine it *was* most
probably arrived at as being the empirically determined optimal compromise
between sound production and survivability of uncontrollable humidity
reactions. "Hell Vito, nothing else has worked, let's try this - Wow, it
works! Sounds pretty good too." Keep all the dimensional fluctuations going
in the same direction and they can be worked around. Seems plausible.


>
>Bob Hohf
>Wisconsin               
>



 Ron Nossaman