Downbearing / crown support question.

[Ric Brekne]

Stephane and Terry

Hi you two. I have always adhered to the idea that ribs have had two 
primary functions. One has been to provide adequate support (which I 
believe can be eliminated at this point) and the other has been to deal 
with the problem of  anisotropism.  So if one can eliminate the need for 
crown/down bearing support then one can use ribs exclusively for the 
later. What I am wondering then is what this would allow the designer to 
do in terms of rib dimensions, number of ribs... etc.  I'm looking for 
thoughts on those issues given an assumed total independence from any 
need to provide crown support.

As for justification for the need to address anisotropism in itself I 
refer to the below from the 5 lectures.  I think there has been a deal 
written on the subject elsewhere as well.


 From Wogram:

    "A stiffening of the soundboard would improve the sound radiation
    efficiency, as a stiff soundboard is less inclined to subdivide into
    small vibrating areas. One purpose of the ribs is precisely to
    stiffen the soundboard, which indeed is thin in proportion to its
    size. Another purpose is to "homogenize" the soundboard by
    equalizing the difference in bending stiffness (elasticity modulus)
    parallel to and across the grain. The moduli of elasticity in the
    two directions are in a ratio of approximately 20:1 (anisotropism).
    If this  is not compensated for by the addition of ribs running
    across the grain, the effective vibrating area is reduced and the
    radiation efficiency is decreased over a broad frequency band."

Cheers
RicB

Hi Ric.

Same reaction here :  why consider ribs if their role in supporting
down bearing force is meant ?  The only accepted theory for now is that
(apart from sustaining the down bearing forces) the ribs help the sound 
wave
traveling across the grain of the panel.  Now, who said that this is
important ?  I believe that the panel (soundboard) must be under some
compression to sound well.  Same for strings : they must have some tension
to sound well.  I think this has to deal with some internal frictions in 
both
cases.  Do you think a board where the ribs sustain all of the down bearing
forces, and a panel 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.

Hi Ric - I don't understand what you are asking. If a panel can support
downbearing somehow, then why the concern regarding rib design? - you
wouldn't need ribs presumably. I'm sure I'm missing something here.....

Terry Farrell


 >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 down bearing completely independent of ribs in a standard
 > single ply panel.... what would the primary design considerations for 
ribs
 > be ?
 >
 > Cheers
 > RicB
 >
 >