>
>My own degree of understanding of these matters still remains at a
>rather young stage, yet one theme comes back again and again in
>these discussions. That is that the sound that the panel is able to
>project is dependent upon its stiffness and mass. Certainly any
>particular combination of these is in itself independent of string
>coupling. John Hartman is correct in saying that down bearing can
>increase the stiffness of the panel without added to the panels
>mass... tho the strings have mass to... and these things have a
>habit of working both ways in some sense or another... Still
>essentially John is correct..........
>
>
>The question remains then can good sound result without any
>particular amount of down bearing. It seems clear to me at any rate
>that the answer to this is not more complicated then asking whether
>a soundboard can be made uniformly stiff enough while being
>comprised of appropriate levels and placement of mass. If that can
>be done without the additional stiffness that down bearing supplies
>in traditional piano design... then so be it. And it would appear,
>given plenty enough <<oddball >> examples of reverse crown and or
>buckling that do not adversely affect sound production that this is
>the case.
>
>On the other hand... the lack of any design that satisfactorily
>would exploit this would tend to counter that conclusion.
>
>Just a few thoughts.
>RicB
I keep reading posts by various people stating that the
soundboard gets 'stiffer' as downbearing is applied. But I have seen
no experimental evidence to support this supposition. The only
experimental evidence that I have seen was that presented by Ron
Overs some time back in which he took a crowned ribbed panel, loaded
it incrementally, and noted the deflections. His data showed just
the opposite; that the panel was getting less stiff as the load
(simulated 'downbearing' if you will) was increased. If someone has
some data to support the phenomenon of increased board stiffness with
increasing downbearing then please share it with us. The only
support for this position that I have heard has been anectodal
stories along the lines of, 'I pushed down on the new board and it
deflected, then I leaned on it with all my weight and it wouldn't
deflect any more, so it obviously was getting stiffer'. It was
reaching a state of equilibrium for the applied load based on the
stiffness of the system. That doesn't mean it was getting 'stiffer'.
There is a difference between preload and stiffness. As you
apply more downbearing load to the board then the preload (or
prestress if you prefer) will increase. It's not hard to believe
that this could have some affect on the vibrational characteristics
of the board and experience tells us that it does. Putting
downbearing on the board usually seems to have a beneficial effect.
Stiffness is a relationship between load and deflection. If two
beams have the same load applied to them, then the one that deflects
less is 'stiffer' for a load applied at that particular spot.
Increasing stiffness under load would mean that there was
incrementally less deflection for unit increases in load. If a beam
deflects down 1/10 of an inch for an applied load of 1000 lbs, then
if it is getting 'stiffer' under increasing load, when you applied an
additional 1000 lbs of load the additional deflection would be less
than 1/10 of an inch. I see no reason why a ribbed panel would
behave in this way. That's not to say that I can't be wrong and that
there's not something about this particular system that I've
overlooked or don't understand. But I'd like to see some
experimental evidence to prove it.
This distinction is important to potential soundboard design.
If the important thing is increasing the stiffness without increasing
the mass, then an alternative soundboard made of something like
honeycomb sandwich might give the desired performance without any
downbearing. If the important thing is preload or prestress in the
panel then the honeycomb panel might be a waste of time and
downbearing would be essential regardless of the type of panel you
used.
Also, if the board is not getting stiffer as a result of
deflection, it raises the question of what function the crown is
performing. If the crowned board is not getting stiffer as it
deflects down, then a flat board would be just as stiff as a crowned
board. So, the reason for the crown would not be 'stiffness'.
Phil Ford
--
Phillip Ford
Piano Service and Restoration
1777 Yosemite Ave - 130
San Francisco, CA 94124
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