More on soundboard crown

[Delwin D Fandrich]

----- Original Message -----
From: "Phillip Ford" <fordpiano@earthlink.net>
To: <pianotech@ptg.org>
Sent: August 15, 2003 11:22 PM
Subject: Re: More on soundboard crown


> Stone is not selected for arches.  Arches are selected for stone.  If
> stone is the material that you have to work with and you have an
> unsupported span you don't have much choice but to use an arch.
> Stone is very good in compression but notably bad in tension (even
> more so the mortar).  The arch allows the load to be carried in
> compression rather than in bending, which would result in tension.
> This doesn't mean that arches can't be built with other materials.
>
> >>The soundboard panel is made of wood
> >>which is far from non-compressible. Even longitudinally. M&H makes much
of
> >>their little demo showing how their spider is supposed to support
crown.
> >>What they don't mention is that it takes only light finger pressure to
> >>flatten the piece out. Nor do they mention that if they leave the piece
in
> >>place for any period of time, say a month or two, the crown disappears.
> >>Wood, at least given the crown radii typical to the piano, is not rigid
> >>enough to function as a structural arch.
> >>
> >>Del
>
>
> 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.
>

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. 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!

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