RC vs CC again

[Richard Brekne]

Delwin D Fandrich wrote:
> 
>  
> > Secondly...we have this unclear bit about panels of same dimensions with
> > ribs of same dimensions and with same crown... in two boards one CC and
> > one RC.  Seems to me that the CC board here is bound to have more
> > stiffness due to higher panel compression. Tho mass and dimensions be
> > the same... in other words... one is able to achieve a higher stiffness
> > to mass level in the compression board... all other things being
> > equal....
> 
> Compression-crowned soundboard systems and rib-crowned soundboard systems
> are two distinctly different designs. They do not use ribs of the same
> dimensions and the same crown. That is the whole point.

I suspected something along these lines Del. Thanks. It's has been
bothering me ever since it came up a couple weeks back. So the notion of
two soundboards...one CC and the other RC,  both with exactly the same
mass, parts dimensions, and crown isnt really going to result in the
same kind of sound at all... because the CC board will have more
stiffness... Is that about right ?... and same downbearing applied to
both will result in a different
match of impedances. 

> The ribs used in a compression-crowned soundboard system must, by
> definition, be somewhat flexible. They must be flexible enough for the
> expanding soundboard panel to force them into a crown. Hence, they are
> nearly always somewhat wider than they are tall. As well they are, again by
> definition, flat when glued to the soundboard panel.
> 
> The ribs used in a rib-crowned system must act as structural beams. They
> are nearly always taller than they are wide. And they must, also by
> definition, be crowned in some way. All of the design crown comes from the
> crown machined into the rib. Yes, there may be some additional crown
> induced by the expansion of the soundboard panel under certain climactic
> conditions, but this is incidental to the design.
> 

Ok... so what is this telling me.... that in order to get two panels,
one a CC
and the other an RC, to have the same stiffness, then something else is
going to have to be significantly different about them. How are you
going to be able to end up with the similiar stiffness to mass ratios
without either having significantly different crown and / or rib
dimensions ? 

> Yes, the two technologies can be blended as is the practice of some. But
> that is a whole different issue.
> 
> Del
> 
> 

RicB
-- 
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html
http://www.hf.uib.no/grieg/personer/cv_RB.html