Soundboard stiffening

[Ron Nossaman]

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>Please tell us all how a panel with a convex upper
>surface can not spread out at the bottom when pressure
>is applied from above, Ron.
>      Doesn't make any sense to me at all.
>      Thump


Thump, and everyone else who cheerfully embraced the latest declaration 
that soundboard crown is a buttressed arch,

Phil Ford and I discussed this in detail on list not too long ago. 
Describing it again most likely won't have better effect than it did the 
last time, so that leaves trying it for yourself. Make a three ribs about 
18" (460mm) long, one flat and two with a crown machined in. Make them 
about 20mm (3/4") square and feather them like you would for a real piano. 
Cut some panel material about 5" (120mm) wide (parallel to the grain), and 
somewhat longer than the ribs. Dry one down to 4%-4.5% humidity and glue it 
to the flat rib. This is the compression crowned example. Dry the other to 
about 6% and glue it to the crowned rib for the rib crowned example. After 
the panels are trimmed square to the rib ends and have reached moisture 
equilibrium with the shop, you should have nice crowns in both models. Set 
up a stiff plank for a base, with blocks of wood (sides cut and squared) 
clamped to it so one of the models will slip between the blocks with zero 
clearance at the ends. Set a spacer under each end of the rib so that the 
bottom of the rib is clear of the plank by at least the amount of crown in 
the model. Now place a straight block of wood nearly the length of the rib 
on top of the panel, and start applying down pressure on it with another 
clamp, depressing the crown of the model. The block of wood on top is so 
you won't press the crown below flat. Watch the ends of the panel as the 
crown is depressed. They WILL pull away from the end blocks. Now do it with 
the other model. Same thing. Now do it with just the crowned rib. Same 
thing. This is exactly backward of the  way an arch would work. Soundboard 
crown is NOT, nor has it ever been an arch because the end supports are not 
below the centroid line of the assembly. They are significantly above it, 
as in a cable suspension, and no, I'm not saying the cable suspension 
retains crown if you keep the rim edges from converging. Crown is still 
formed and maintained by the rib and panel. The rim has nearly nothing to 
do with either formation, or retention of crown.

Ron N

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