Soundboard stiffening

[Phillip Ford]

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>
>    Phil writes
>
>       I keep reading posts by various people stating that the
>soundboard gets 'stiffer' as downbearing is applied....
>
>  >> I believe it was getting stiffer but remember we're talking 
>about a light weight system with limited ability to carry loads so 
>great as to crush them altogether.
>
>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. 
>
>  >>I'm honestly not sure about that. It seems like to me it's both.
>   Was Ron Os demonstation on a board glued to the case liner? 
>For the board will certainly react differently if the test is out of 
>the piano with out the edges glued down

It was not glued to a rim.  But it was clamped so that the edges 
could not move out.  It should be in the archives.

>
>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.
>
>  >> And too much makes for a stingy sound especially in the treble

Yes.  Good point.  If it was just about increasing stiffness while 
not increasing mass, then it would seem that one wouldn't so quickly 
reach an upper limit.

>
>
>      Stiffness is a relationship between load and deflection....
>
>    This is exactly what a board is doing when it is being compressed 
>up to a point & then it will fail just as the beam will.
>
>   I see no reason why a ribbed panel would
>behave in this way...
>
>  >>The way I look at it is that putting boards under some modest 
>amount of strain via compression or tension for that matter 
>makes the board more reactive and tonally efficient because of the 
>impeding effect, which moves air & thet's why soundboards are built 
>this way. If it wasn't Ron O would be building a flat board.

This gets back to one of my original questions.  Is it increasing 
stiffness that matters or stress (or strain as you put it) in the 
board?  Or something else altogether?

>
>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
>  >> But I think it is getting stiffer.
>      Dale

Well, you've done it many times, so your opinion means more than 
mine.  But if it is getting stiffer I'd like to know why.  Have you 
tried the same thing with an initially flat board?  Did it get 
stiffer as you wedged it down?  Was the increase in stiffness 
comparable to a crowned board?  If so, then why bother with the 
crown?  I think it's time for some measurements.  I guess I'm going 
to have to do some deflection vs. load measurements on a few pianos. 
Another thing to put on the list of things to do.

Phil
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