More CC vs RC questions was RE: Killer Octave & Pitch Raise

[Sarah Fox]

Hi David,

> Well this is an important point.  Why doesn't compressing the panel make
> it stiffer?  It seems like it must?  Even if compression and stiffness
> are not synonymous in this discussion, then the question is does
> compression itself change the physical and acoustical properties of the
> panel?

Quick question, quick answer -- I hope...  then with more complexities 
thrown in...

I think you're confusing stiffness with force.  "Stiffness" simply means 
"spring constant."  If you take a relaxed spring and compress it by a mm, it 
takes a certain amount of force to do that.  Then if you compress the spring 
by 10 mm and add enough force to compress it a mm more, it will take the 
same increment in force to compress it that last mm as it took to compress 
the relaxed spring by the first mm.

Now consider the vibrating string.  To simplify GREATLY, as the string 
vibrates up and down, it exerts up and down forces on the bridge, which 
exerts those same forces on the board.  Irrespective of the total amount of 
tension on the board, the amount of up and down travel will be the same.  To 
unsimplify, this assumes the board is an ideal spring, which it is clearly 
not.  It also applies a static argument to a dynamic situation.  It also 
makes certain assumptions about force directionality that may or may not be 
true.  It also completely ignores interaction between the string, bridge, 
and board -- circle of sound stuph.  All that notwithstanding, what we're 
still talking about with regard to board stiffness is what *increment* in 
force is needed to move the board by what increment.

There's another caveat.  If the panel has cracks in it, then the stiffness 
declines.  In the vicinity of the crack, the stiffness is attributable ONLY 
to the underlying ribs.

And still another thing to note:  In an RC/S board, the panel *does*, in 
fact, provide stiffness, in much the same sense as a tensioned CC panel. 
It's just that so much more stiffness is provided by the ribs.  When the 
RC/S board is put under load, the panel *is* compressed, just not as much as 
a CC panel.  The primary difference would be that the *less* compressed RC/S 
panel might be a bit more elastic than the compressed CC panel and therefore 
might gain efficiency.  Perhaps also the RC/S panel is thinner than the CC 
panel???

Of course when an RC/S panel cracks, there's still going to be a loss in 
stiffness.  This loss could be reduced with judicial selection of materials 
in the ribs.  Consider a rib with two different woods laminated together. 
The layer of wood adjoining the panel is the stiffest.  The wood farthest 
away from the panel is less stiff.  Thus, the compression-neutral part of 
the rib is closer to the panel, reducing the mechanical advantage of panel 
compression on the flexure of the rib.  (Hmmmmm???)  That would give the 
assembly more long-term stability.

Peace,
Sarah