Downbearing / crown support question.

[David Love]

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
 



And Ric B wrote:

“
It is interesting to note that he sites 2 primary acoustic functions of 
the ribs.  One is to increase soundboard stiffness as a whole. It is 
this bit that is interesting with regard to the RC& C vs CC discussions 
to my mind.  It is clear that placing the panel under compression cross 
grain will increase its stiffness to a significant degree.  This is 
especially valuable because the increase comes about without any 
additional mass.  That is to say, one adds mass when one adds ribs,  but 
if one figures panel compression into the equation one gets extra 
stiffness for the same amount of total mass.  I.e.  a compression 
soundboard with the same set of ribs will have more stiffness compared 
to a non CC board all other things being equal.”  

Stéphane Collin wrote:

Hi Ric.

Same reaction here :  why consider ribs if their role in supporting 
downbearing force is neant ?  The only accepted theory for now is that 
(apart from sustaining the downbearing forces) the ribs help the sound wave 
travelling across the grain of the pannel.  Now, who said that this is 
important ?  I believe that the pannel (soundboard) must be under some 
compression to sound well.  Same for strings : they must have some tension 
to sound well.  I tink this has to deal with some internal frictions in both

cases.  Do you think a board where the ribs sustain all of the downbearing 
forces, and a pannel freely posed on these ribs, without bearing any force, 
would sound good ?  My answer is not, but I never tried this at home.

Best regards.

Stéphane Collin.


----- Original Message ----- 
From: "Ric Brekne" <ricbrek at broadpark.no>
To: "pianotech" <pianotech at ptg.org>
Sent: Friday, June 16, 2006 9:04 PM
Subject: Downbearing / crown support question.


>I suppose most of the rebuilders are already on the road to Rochester... 
>but I thought I'd ask anyways....
>
> Assuming for the moment that one could provide adequate and total support 
> for crown and downbearing completely independant of ribs in a standard 
> single ply panel.... what would the primary design considerations for ribs

> be ?
>
> Cheers
> RicB
>
>