RC vs CC again

[Delwin D Fandrich]

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "Pianotech" <pianotech@ptg.org>
Sent: October 02, 2003 8:13 AM
Subject: Re: RC vs CC again


>
>
> It appears like they cant have all the same physical characteristics
> period. Somethings always got to be different, simply because ones got
> compression forceing crown, and the other has ribs doing it.

Bingo!


>
> Well.. not in the case of identical panel thickness, identical rib
> dimensions, same mass, and identical crown.  The CC board simply has to
> have more stiffness as long as all that remains the same... yes ?
>
> Beyond that... its open season as it were...

So, stop trying to force the two into one structural mold. They don't fit.
They represent two different paths to a similar, if not identical end. At
least they can if that is what designer/builder is after.


>
> I got that bit... thats been clear for about a year now.... it was just
> this identical bit that came up... seemed to different things said by
> different people... chalk it up to slight problems in communication :):)

So, ignore the concept. It isn't going to happen. Although at this point,
if I were really sadistic, I'd remind you that many, if not most, of the
pianos on the market use hybrids -- some combination of the two systems
which blend the properties of each and which make distinctions between the
two rather murky.

It's easier to sort out the extremes first.

A pure compression-crowned soundboard system depends entirely on the
compression within the panel (and the resulting rib-to-panel stress
interface) to form and maintain crown and to support string bearing. The
ribs are flat when they are glued to the panel, i.e., they have no crown
machined in them, and they resist the formation of crown. The system can be
made stiffer by using a thicker soundboard panel which obviously also
increases the mass of the system. It can also be made stiffer -- at least
temporarily -- by drying the panel to some lower moisture content prior to
ribbing. There are obvious limits to how dry one can make a wood panel, so
the same effect can be obtained by keeping the panel at a somewhat higher
moisture content and pressing flat ribs into curved cauls. Both increase
the rate of compression-set within the panel.

A pure rib-crowned soundboard system depends entirely on the rib both to
form crown and to structurally support string bearing. Both stiffness and
mass are controlled by manipulating the rib's height-to-width ratio. The
soundboard panel is not load-bearing in this pure system so in theory its
thickness doesn't matter. In fact, of course, the panel does have both mass
and stiffness and its thickness affects the apparent stiffness felt by the
bridges through both the treble and the low bass.

Everything else is a hybrid system of some sort.

Del