Downbearing / crown support question.

[Ric Brekne]

David


1. Nobody claimed anywhere that RC&S boards have more or less mass then 
CC boards. In fact the example given specifically specified the 
opposite. What was claimed, and is true enough is that all other things 
being equal a CC board will be stiffer then an RC&S board.

2 . I see no where anyone lumping all of any kind of boards into any 
general category and indeed what does any of that have to do with the 
original question posed ?

3. Ribs in a CC board do indeed support crown, tho not as beam support. 
The perspective that it is only the panel compression that supports 
crown is one many share, but it has been shown to my satisfaction that 
this is just not true. It is a combination of ribs tension and panel 
compression working against each other that creates the stresses in the 
assembly which support crown. The very fact that the ribs actually do 
<<bend>> show conclusively that ribs become tensioned in the top half, 
and that indeed tho the (crowned) panel is under significant compression 
is also has grown in size at the interface between it and the ribs since 
all this is above the line of centroids for the combined assembly. The 
ribs act much like a cable support. The more stress there is due to 
either down bearing or increased humidity, the more counter stress 
(support) the system creates. This works until the weakest link in the 
chain fails... ie panel compression failure. The fact that the ribs in 
themselves provide an additional stress in the opposite direction of 
crown is only one part of the entire picture. Tho ribs in a CC board are 
not beam supports, they most certainly do support crown in their 
function as part of a CC board.

None of this has anything to do with what I originally asked about and I 
certainly was not looking to get an argument started about CC vs RC&S 
boards started. I'm just looking for constructive thoughts on what 
soundboard builders / designers would do with ribs if they didn't have 
to figure crown support into the equation.


Thanks.
RicB



Not sure for the reason to speculate on the sound of something you've never
heard. A string under tension versus a soundboard under compression is not
the proper analogy. A better analogy would be to compare it to the
soundboard assembly stiffness.

There are plenty of examples out in the field or RC&S boards to listen to.
Not only some of those who will be showing there efforts at the convention,
but many builders are building a radius into the ribs and gluing up in the
6-7% EMC range, i.e. minimal to no compression. I have heard many RC&S
boards in various iterations and I can say first hand that they sound good.
I can also say that RC&S is only the format, there are many different
possibilities within this group in terms of scale choice, grain angle and
density, rib dimensions and number, rib layout, rib material, etc., etc..
To lump all RC&S boards (as compression boards) into one general category is
naïve. I had a chance to tune the Charles Walter 6'4" grand that was at the
NAMM show today. This is a Fandrich design, RC&S board, full cutoff,
beautiful semi log scale throughout, totally transparent through the breaks,
vertical hitches. The owner, a mature concert level player, bought it
because he found the tonal irregularities of Steinways objectionable.

With respect to Ric B’s comments below, RC&S boards have no more mass in the
ribs, typically, than compression boards. The orientation of the ribs
changes from shorter and wider to taller and narrower, but the total mass in
not changed significantly, at least not that I have seen. Also, the ribs on
a compression board do not provide support. This has been stated many
times. If you bend the ribs by virtue of compression the panel, then the
force of the ribs will be pulling downward not pushing up. The bend of the
ribs plus the downbearing force from the strings is being supported by the
compression in the panel--that does require some stiffness. The ribs do
contribute to stabilizing the panel across the grain, and no compression
style board (or any other for that matter) would have a chance of any type
of stability without ribs running across the grain. Of course, you are
right when you say that “a compression soundboard with the same set of ribs
will have more stiffness compared to a non CC board all other things being
equal”. But so what? All other things are not equal when you build the
ribs to support and add stiffness by virtue of their shape, dimensions,
orientation (vertical vs horizontal), and layout. So I’m not sure what your
point is.

We keep going back to the same old thing. The best thing for y’all to do
would be either build an RC&S board or listen to several and then at least
your armchair speculation will somewhat less padded.

David Love