Soundboard crown

[Richard Brekne]

Sarah Fox wrote:

> 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.

For some basic stuff on this, which I am sure you will be able to whip right
through...   See the 5 lectures on Piano Acoustics edited by Askenfelt. Conklin,
Wogram, and that gang of bonified Music Acousticians actually did some real
science on the matter. Interesting reading for anyone really, tho some of us
have to brush up a bit on what we were supposed to learn in all those high
school classes to relate to some of this.


> 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???  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.)

If I remember right, the lower resonate frequency modes are adjusted fairly
predictably upwards with the advent of string bearing, or rather... when the
panel is further stiffened by the downward pressure from the string plane. Above
2000 hz (again if I remember correctly) the shift in mode frequency is less
pronounced or negligable. I suppose I will have to break out that article again
to double check this.


>
> 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?
>
> Has anyone ever played around with carbon fiber?  That might behave a bit
> more like wood, yet without the problems associated with varying MC.
>

Thats been aired a few times. Perhaps it might function well. I guess we will
know when somebody tries to build one this way. Gee.... I wonder if it will be
Steinway... or lets see... who else ????? grin.

>
> Peace,
> Sarah
>
> 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.  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.  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.

 Interesting.

Cheers
RicB

--
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html
http://www.hf.uib.no/grieg/personer/cv_RB.html