Soundboard crown

[Sarah Fox]

Hi Del (and Ric),

> > In addition to stiffness arguments, I wonder about the impact of a crown
> on
> > the vibrational mode of the board.  If a domed panel is deflected at one

<snip>

> Interesting point. And this, of course, is the motivation behind the
> various soundboard cutoff bars. They make possible the placement of the
> tenor bridge at least somewhat closer to the middle of the board.

Thanks!  That makes sense!

> > factor alone.  Perhaps a crowned soundboard would be predisposed to
enter
> > into higher vibrational modes, responding better to higher
frequencies???
> > (Just guessing.)
>
> In real life there are so many frequencies being simultaneously generated
> by the strings it would be hard to measure or quantify any of this--let
> alone predict it. But it does illustrate the incredible complexity of the
> system we work with. How am I supposed to design one of these things when
I
> don't really know anything about them?

Well, there's modal analysis, but I presume that assumes linearity in the
system, which probably breaks down, especially at higher amplitudes.  I can
appreciate the complexities of this system.  I once did some preliminary
research on an acoustically simpler system and couldn't land an NIH or NSF
grant to study it further, using something very similar to a modal analysis.
Some reviewers seemed to lack confidence that it could be understood via the
techniques I proposed.  I think they were being overly pessimistic; however,
I'll admit the acoustic complexities of the system were daunting at times!

> > Has anyone ever played around with carbon fiber?  That might behave a
bit
> > more like wood, yet without the problems associated with varying MC.
>
> Yes, but not as much as I'd like....

????

It doesn't behave as much like as wood as you'd like, or it hasn't been
studied as much as you like???

I've seen discussions on this list about soundboards vs. boat hulls,
obviously from boating enthusiasts.  It would seem that someone on this list
would be into both piano rebuilding and boat repair/manufacture -- someone
with experience with fiberglas hulls.  Carbon fiber isn't conceptually that
much different from fiberglas, as I understand it, provided one can find the
requisite fiber and resin materials.  Somebody really ought to try this out.
Imagine a synthetic soundboard with integral ribs and bridge.  No more rib
or bridge separation.  No more cracking around bridge pins.   :-)

> > PS My initial, noninvasive, cursory experiments with mass loading of the
> > killer octave region seem to alter the spectral properties of the
> sound --

<snip>

> How are you mass loading the thing? Where, how and how much can make quite
> a difference.

Well, as I said, it was "cursory" and "noninvasive" stuff -- hardly a
satisfactory test.  I had a few moments before I had to scoot out of town,
so I rubber-banded a large (heavy) magnet to the top of the high-treble
bridge (around C7), making firm, non-buzzy contact with the tops of the
bridge pins.  I don't know the mass of the magnet, off hand, but it was
heavy.  My conceptual goal was to maximize bridge inertia and therefore
maximally prevent energy transfer from the strings to the bridge -- with the
expectation that sustain would be lengthened at the cost of amplitude.  I
was looking for an obvious, but not necessarily desirable effect, which is
exactly what I got.

(Ric, Del, my interest in soundboard mechanics has been rekindled.  I think
I'll give those 5 lectures another go-through.  Fascinating stuff.)

Peace,
Sarah