Interesting Old Soundboard

[Ron Nossaman]

> Six millimeters thick 
>along the long bridge, and thinned to 4.5 mm in the area of the bass bridge. 
>Is this unusually thin? 

Yes it is, at least from what I've seen.


>It had kinda hefty ribs, and quite some arc to them. 

Hefty wide, or hefty deep? It do make a difference, you know.


>After taking the ribs off the board, I see that most of them have about a 35 
>to 40-foot radius curve to them. Amazingly tight. And they must have been 
>subject to a fair amount of compression crowning also because the back side 
>of the ribs is curved quite a bit also, unless of course they cut/plane them 
>that way. I thought all the old timers used 60-foot arcs?

I've heard this for 25 years, and I'd really like to know where that 60'
figure came from, since it only seems to appear as a relatively recent
after the fact - post mortem guestimation of what someone (???) thought
might have been the intent of the original builder(s). With so many of the
old pianos having compression crowned soundboards, and the resulting crown
"radius" under string bearing load being entirely dependant on the panel
density and thickness, MC of the panel at assembly, scale tension and
bearing angles, number, dimension, and placement of ribs, the relationship
of resulting crown radius to intended crown radius (assuming that there was
an intended crown radius in the first place) can't be anything but random.
On the other hand, if a specific tight radius rib crown was chosen as a
design parameter, machined into the rib, and a thinner than "usual" panel
was chosen because the high crowned rib was expected to carry the load that
the severely over compressed panel would normally have had to suffer with a
compression crowned board (so the same load carrying capacity could be
achieved while staying well under the material stress loads of that
"conventional" compression crowned board), the result would be a stronger,
lighter, more responsive assembly with more nearly predictable load
deflection characteristics and longer expected service life. The mystery
shallows.

>Boy, I can see that one could spend a lot of time on the back of soundboards 
>getting to know them.
>
>Terry Farrell
  
You betcha, and it's fascinating stuff.

Ron N