Soundboard stiffness variances

[Richard Brekne]

Rolf... Heisenberg's particle ???..  you guys crack me up !

The stiffness you are trying to calculate (who said anything about 
measuring ?)  is nor more or less of a moving target then with a RS&C 
system.  Its just generated by different mechanisms.

A CR board in unloaded state have pretty predictable amounts of 
compression in them. No problem there.  The problem seems to be 
understanding what exactly happens when you load them. I would submit 
that a good starting place is to view the curve of underside of the 
unloaded panel as if it was unrestricted by the ribs and figure how much 
the edge to edge distance would increase if it were flattened thus. Then 
one has a good idea of how much added compression is created in the 
panel for any degree of load since since that translates to some 
specific lessening of its radius. Knowing how much the edge to edge 
distance would increase if unrestricted yields the amount of compression 
stress generated in the restricted case.  Added to what is already there 
from the unloaded state and one can figure pretty accurately what amount 
of compression is in the panel for any given amount of loading. One can 
easily calculate how much resistance towards bending a given rib has for 
a given starting point. In a CR board in unloaded state the ribs already 
stressed and bent in a convex (to the panel) form. Downbearing will 
exert at least two forces on the ribs.. long the grain axial stress 
which will tend to bend them further, and a downwards force which will 
tend towards straightening them out.  Since up until the point panel 
compression failure occurs it is obvious which one of these two wins 
out.... its just to calculate the net effect of each of these two 
opposing forces and one should be in the ball park.

One other point would which seems to be over looked is the shape (convex 
or concave) of the rib relative to the panels underside at any given 
time.  But I'll leave that lie for the moment.

Heisneberg's Particle indeed... quantum physics like realllly applies 
here :)

Cheers
RicB

    Well put, David. My instinct and experience would lead me to agree.
    It would sort of be like measuring Heisenberg's particle in that the
    very stiffness we would try to measure is always changing, just by
    our very breath as we lean over the thing to examine it.

    Jude Reveley, RPT
    Absolute Piano Restoration, LLC
    Lowell, Massachusetts
    (978) 323-4545