rib glueing and panel compression

[Ron Nossaman]

>Hi Ron
>Enjoyed  your usual succinct and clear technical thoughts on the subject of
>recent interest on the list.I n your  current catalogue of various
>soundboard forces senarios you mentioned that in the pure forced crowned
>board, only the compression of the panel is creating the crown and that the
>ribs being flat are actually resisting that effort which I agree but as to
>downbearing I have this thought. I no longer think that the compression of
>the panel is the only force opposing the downbearing in this type of
>construction..I have heard that some who use this method allow the boards to
>puff up a bit before glueing them to the rim which does bend the rib further
>I,m thinking 40% R.H..Once that bent rib is anchored in the case the ends
>can no longer move as much..It,s true that they can unbend some of course as
>the panel shrinks or cracks or even self destructs  but I have so often
>removed boards the had some crown with strings removed  that once off the
>rim totally flattened out . My point being that those force crowned ribs
>once glued in position where an essential  part of the equation in
>contributing to the resistance of downbearing and also because ofthat making
>tonal contribution.what do think ?Also all those pianos from the past we
>admire so much all used a procedure we all no longer think wise to embrace.
>But wow they really sounded great.! sincerely Dale Erwin

Dale, you got me. It has to be true that there is some leverage support
from the rib ends mortised into the rim if the bedding angle is as steep or
steeper than the  "landing" angle at the ends of the ribs, and the ribs are
solidly bedded and glued to the rim. It's a good point, and I should have
been more thorough and detailed. However (you must have known there had to
be a "however", didn't you?) The same situation would apply with a rib
crowned assembly too, and I'd sure hate to have to depend on and try to
anticipate the effects of this in the design. We have all heard absolutely
LUSCIOUS sounding pianos with compression crowned boards, and a lot of
truly WRETCHED sounding ones as well, and that's pretty much a distillation
of the campaign for rib crowning. Relative predictability and some design
control of the final acoustic characteristics of the board, dependable
longevity of the system, and some freedom from the dependence on the
density and EMC reactivity of the panel are the benefits of rib crowning.
With a decent design, the overall sound can be kept more toward the
"luscious" end of the spectrum, and with a narrower statistical deviation.
That's for me!


Ron N