Soundboard crown

[Delwin D Fandrich]

----- Original Message -----
From: "Sarah Fox" <sarah@gendernet.org>
To: "Pianotech" <pianotech@ptg.org>
Sent: August 11, 2003 11:09 PM
Subject: Re: Soundboard crown


> Hi Del et al.,
>
> Fascinating discussion!
>
> 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
> point nearer to one edge, the opposite end of the board would deflect in
the
> opposite direction, with a nodal point in the middle -- more or less.
> Correct???

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.


>
> Now consider the same board, flattened:  A deflection nearer to
> one edge would result in a deflection in the same direction over the
> remainder of the board, to greater or lesser degrees.  Thus, at a
frequency
> of zero, the board would "vibrate"/flex in a bimodal pattern (correct
term?)
> with a crown and in a unimodal pattern with no crown.   I'm not saying
that
> the board would be incapable of other vibrational modes at other
> frequencies, which of course it would.  I'm merely suggesting that the
> vibrational properties of the board would certainly *change* from this
> 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?


>
> The spector of manmade materials has also been raised, another subject I
> find fascinating.  I particularly enjoyed hearing an audio file of a
piano
> with a steel soundboard in the five/six/seven (can't remember) lecture
> series.  The piano had a beautiful, full, rich bass, well defined tenor,
and
> a bizarre, pingy, ringy, un-piano-like treble.  I was left wondering
whether
> the soundboard was too efficient at higher frequencies, not
> absorbing/damping vibrational energy as badly as wood -- or whether the
> soundboard had other sorts of mechanical differences.  What exactly were
the
> *functional* differences between this board and a wooden one?

As was the sound of a piano having a foam core/metal faced soundboard. In
that case the soundboard was very light and stiff. I don't know anything
about the construction of the soundboard used in the John Challis piano.


>
> 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....


>
> PS My initial, noninvasive, cursory experiments with mass loading of the
> killer octave region seem to alter the spectral properties of the
sound --
> *somewhat* longer sustain, but kinda pingy.  Interesting.  I think the
> "pingy" nature of the sound comes from a frequency dependence of energy
> transfer from the string to the bridge, hence different decay rates for
> different partials.  I'm also finding, I believe, that a very large part
of
> the responsiveness in this region is attributable to the properties of
the
> *entire* soundboard, not just the top end.

Well, certainly that portion of the soundboard between the bridge and the
rim. With a compression-crowned soundboard this area fairly rapidly looses
its ability to support crown, hence its stiffness decreases rather quickly.
With the panel compression dissipated, even with added springs and mass it
is still a rather inefficient transducer. And you're right about the
frequency dependence of energy transfer affecting the decay rates of the
various partials. (It's why hammer voicing sometimes has a positive effect
on sustain in the killer octave region.) Adding in a bit more mass should
reduce the "pingy" effect, though it will probably come at the expense of
volume.

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


>
> After a short initial period,
> vibrations from the string are dissipated, exciting harmonic frequencies
in
> other strings throughout the piano -- perhaps especially in the
backscale?
> If I try to damp vibrations in the high treble strings with my finger, I
can
> only do it within a short timeframe after the attack (progressively
shorter
> for higher notes).  Thereafter, there is still sustain, but it cannot be
> damped, because the vibrations have been excited throughout the piano,
while
> the vibrations of the note's three strings have all but dissipated.

Most of this is the result of the undamped backscale being excited by the
motion of the bridge.


>
> I think
> this "whole board response" keeps the mass loading of the upper end from
> having any more of an effect on the overall sustain of a high treble
note.

Well, the mass loading is only a substitute for the springiness that has
been lost. And it is only a part of the equation.

Del