More on soundboard crown

[Phillip Ford]

>
>  > I agree that if I was designing a panel to work efficiently as an
>>  arch I would use much smaller radii.  However, if I was designing a
>>  rib to work efficiently as a beam I wouldn't make it as wide as it is
>>  tall and I wouldn't have the same cross section over most of its
>>  length.
>>  Phil F
>
>And I agree that structural arches can be made of wood. But not with any
>radii suitable for use in a piano soundboard. All of the examples you've
>come up with have quite a significant radius. And they are hardly shaped
>like piano ribs. Your examples notwithstanding, the structural arch
>principle does not apply to the piano soundboard rib. Relative to its
>length and radius, and especially the relatively small cross-sections at
>the two ends of the rib, wood is simply too easily compressed. Even
>longitudinally.
>
>This idea keeps coming up, mostly, I suppose, because its defenders are not
>willing to actually try it out for themselves--preferring to believe
>legend, tradition and marketing hype to real-world logic and proof.

I prefer to think of it as exercising my own judgement based on 
whatever knowledge I have about structures.   And that judgement says 
that some arch behavior seems feasible.  That's not to say that I 
can't be wrong.  I just want to see more physical proof before being 
convinced.  In spite of what's often implied on this list, not every 
opinion is based on marketing hype.

And I agree that a simple experiment done by me would be the best way 
of convincing me one way or the other.  I don't think I'm unwilling, 
I just haven't done it.

>  The
>next time you have a M&H rim sitting around with its soundboard removed,
>force a typical crown into a rib and glue the thing in! Once the glue has
>dried and/or cured take away whatever mechanism you used to force the crown
>and see what happens. (Lacking a M&H with its soundboard removed you can
>always cobble up a suitable test fixture on a really sturdy workbench. Feel
>free to cheat and make sure the two end buttresses are completely
>rigid--unlike the rim of a M&H which, even with its spider, still has
>some--albeit slight--flex to it. Just make sure the rib is sized, shaped
>and crowned like a real-world piano rib.) It should be quickly and plainly
>obvious to even the most stubborn skeptic that a typical soundboard rib in
>either the M&H rim or the test fixture is not capable of functioning as a
>structural arch. To complete the experiment, assuming for some reason your
>rib doesn't instantly collapse like mine did, you can put an appropriate
>load on it and toss in a few months of compression-set...you'll redefine
>the meaning of reverse-crown!


Now this is more convincing.  I'll give it a try when I have a free 
moment, which should be any month now.

>
>And the ribs we use are not as wide as they are tall nor do they have the
>same cross-section over most of their length. They are (nearly) always
>taller than they are wide and they taper out from the center. One of the
>several advantages of the rib-crowned soundboard system is that you can
>size, shape and form each rib to achieve most any variety of load-carrying
>and acoustical results.
>
>Del

Are they tapering out from the center in height or width?  Care to 
share a picture?

Phil Ford