More on soundboard crown

[Ron Nossaman]

>>In a rib crowned and rib supported board, maybe 95%-100% of the stiffness 
>>comes from the ribs. In a compression crowned board, probably somewhat 
>>over 120% of the stiffness comes from the panel, since the ribs supply 
>>negative stiffness and crown support. A rib crowned but panel supported 
>>board is somewhere above 0%, and under 100%.
>
>I'm sorry but I am not swallowing this one. This is very misleading. If 
>this were true a strip of cross grain soundboard panel say 5" wide and 30" 
>long with a rib glued on using the panel crowning method would have the 
>same stiffness (or less) than the panel alone. This is just not true.

Of course it's not true, and I said no such thing. A compression crowned 
assembly's panel is already supporting whatever load is required to bend 
the ribs from straight to crowned before a gram of string bearing load is 
ever applied. Therefore a panel in a panel crowned board under string 
bearing load is supplying over 100% of the spring resistance necessary to 
provide what we'd call stiffness in the assembly. That's not a theory, 
that's a fact. The bent rib is a built in pre-load, supplying negative lift 
- it's trying to pull the panel flat - but the resulting stiffness comes 
entirely from the panel compression. The rib provides no positive spring 
resistance (to string load) of it's own until the board goes concave - 
often soon after stringing.


>Using your theory we can draw all sorts of faulty conclusions.

Apparently.


>I am going to go out on limb here and predict that from a stiffness point 
>of view it doesn't much matter whether the crown is formed by the shape of 
>the ribs or whether the crown is formed by an expanding panel. The main 
>factor when it comes to stiffness is the cross section and length of the ribs.

You know better than that. Take that 5"x30" piece of panel, and two more 
just like it. Dry two down to 4%MC, and leave the other at equilibrium in 
the shop. Glue a flat 20mm wide x 20mm tall rib on the back of the dried 
down strip, and a 20mm wide x 20mm tall crowned strip on the strip at shop 
EMC, and another crowned rib just like it to the last panel dried to 4%MC. 
Let them all reach the shop EMC. All three assemblies have equal length 
ribs of identical cross sections. Do you really believe they'll be of 
identical, or even remotely similar stiffness? Note that, as with a 
soundboard assembly in a piano, we're talking about stiffness in only one 
direction. Of the three, the rib crowned assembly with the non-compressed 
panel will be the least stiff, the compression crowned with straight rib 
will be stiffer, and the combination compression and rib crowned assembly 
will be stiffest.


>There are many advantages to rib crowned soundboards (God knows we've 
>heard plenty)

There sure are, and I think I recall hearing some from you. Don't you 
redesign Steinway soundboards with crowned ribs when you replace them?


>but the kind of things reported above just muddies the water with absurd 
>claims. You make it sound like a panel crowned soundboard will be as 
>flimsy as an un-ribbed panel.

The claim is an observation, and is quite valid, though it may be 
interpreted unreasonably.


>And what the heck is negative stiffness?

In this case, negative crown support. I had assumed we were talking about 
resistance to string bearing load and calling it stiffness. My 
understanding and use of the nomenclature could very well be inaccurate.

Ron N