R,C&S question

[John Delacour]

At 11:32 +1100 26/1/08, Ron Overs wrote:

>... Here's an image of the soundboard for no. 6, just before it was fitted.
>
>http://members.optuszoo.com.au/ronovers/NO6.14.jpg

Certainly a fine piece of workmanship, like all the pictures I've 
seen of your work.

>These boards, with the piano strung and at pitch, remain crowned 
>enough for a string line across the underside of the mid-panel to 
>nearly clear the height of the soundboard buttons. Crown remains 
>over the entire area of the soundboard, without any of the typical 
>overloading at the bridge 'lines' and the perimeter of the 
>soundboard.

The question in my mind all through this is "to what purpose?".  As I 
understand the RC&S method, as explained by Ron Nossaman in postings 
three days ago, the ribs are so sized and shaped that they will 
"support" the down-bearing of the strings independently of any force 
exerted, through compression or tension, by the soundboard "panel". 
I understand that this (the board) is designed to be in a state of 
equilibrium, relatively speaking, without internal tension or 
compression.  Some compression of the board is in practice mot likely 
to occur as humidity rises, but this is not regarded as a design 
feature -- it happens, it does no harm, but it does not contribute in 
any way to the efficient function of the soundboard or improve its 
ability to transmit and enhance the vibrations forced into it.

By contrast, other methods shrink the board with the main purpose of 
achieving considerable compression across the grain of the board in 
the strung piano, this compression being an end in itself apart from 
its other mechanical functions.

Now if we imagine one of your structures with a hypothetical board 
that is truly neutral and passive (not developing any compression or 
tension) then when you apply the down-bearing of the strings to the 
bridge, your ribs will be pressed downwards until the compression at 
the top of the ribs and the tension at the bottom balances the 
downward force of the strings and the system is in equilibrium.

If we now factor in a degree of expansion for a real board, then 
there will be an increased upward force against the strings as the 
internal compressive forces in the board (panel) take effect with 
increased humidity, BUT in the RC&S case this is incidental.  My 
point is that whatever expansion there is, no matter how slight or 
insignificant, will tend to increase the upward curvature of the 
board and not to decrease it.

So, finally, what then is the need or the virtue of this considerable 
final crown on the soundboard, or any at all.  Why do you not design 
the ribs so that when everything's up and running the soundboard is 
flat?

JD