Soundboard stiffening (was Re: No downbearing)

[David Love]

At the risk of oversimplification, isn't a crowned soundboard just a big
spring, the more you compress it, the more rigid it becomes?

David Love
davidlovepianos@earthlink.net


> [Original Message]
> From: Phillip Ford <fordpiano@earthlink.net>
> To: Pianotech <pianotech@ptg.org>
> Date: 2/1/2004 12:50:17 PM
> Subject: Soundboard stiffening (was Re: No downbearing)
>
> >
> >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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