Soundboard stiffening

[Ron Nossaman]

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>> >>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=20
>> board will certainly react differently if the test is out of the piano=20
>> with out the edges glued down
>
>It was not glued to a rim.  But it was clamped so that the edges could not=
=20
>move out.  It should be in the archives.

I sort of remember this, but not the details. I'll try to find it.=20
Incidentally, the edges won't move out - they'll move in as the board is=20
depressed. It's not an arch, it's a cable suspension. Remember?


>> >> And too much makes for a stingy sound especially in the treble
>
>Yes.  Good point.  If it was just about increasing stiffness while not=20
>increasing mass, then it would seem that one wouldn't so quickly reach an=
=20
>upper limit.

So how much of that is due to restriction from the short back scale lengths=
=20
of a tuned duplex, and how much is the soundboard?


>This gets back to one of my original questions.  Is it increasing=20
>stiffness that matters or stress (or strain as you put it) in the=20
>board?  Or something else altogether?

This is a semantics problem, more than a mechanical one. Take two=20
soundboards, one with a low crown and stiff enough that it deflects 1mm, to=
=20
produce a 1=B0 bearing angle at a certain point in the scale. The other has=
 a=20
higher crown, but is less stiff (lower spring rate), so that it deflects=20
3mm to produce a 1=B0 bearing angle at that same point in the same scale.=20
Both are supporting identical loads at identical heights. Which is stiffer?=
=20
Depends on whether you're talking about load capacity, or spring rate.

Why would a soundboard assembly spring rate get higher as it is loaded?=20
Panel compression will make the difference, in my opinion. I've done=20
deflection tests on model assemblies consisting of one rib and a strip of=20
panel, both compression crowned and rib crowned. The spring rate does=20
increase as the board is deflected. It's not linear like a beam, at least=20
in my testing.

>>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 if the panel compression is what is responsible for the increasing=20
spring rate, and the board is already flat, that means the panel is already=
=20
too crushed to bend the rib into a crown, much less provide the compression=
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resistance to increase spring rate as it is loaded.


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

Me too.

Ron N

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