Soundboard stiffening

[Phillip Ford]

Dale Erwin wrote:
>>> >>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

Phil Ford write:
>>
>>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.

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

I beg your pardon.  I should have said that the ends were clamped so that 
they could not move.  Here's the link to the archived post:

https://www.moypiano.com/ptg/pianotech.php/2003-August/140343.html

Unfortunately, it looks like the words are there, but the data has been 
stripped off.  Perhaps Ron Overs still has that info and would be willing 
to share it again.



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

I don't know.  I suppose one possible way to find out would be to string a 
piano with no downbearing and then apply increasing amounts of downward load on the
bridge with with some sort of loading devices, which would essentially take the back
scale angle out of the picture.



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

I would be talking about spring rate.  I don't think of load capacity as 
'stiffness'.


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

Interesting.  This is the opposite result obtained by Ron Overs, if I 
understand correctly.  I also don't understand what you mean by panel 
compression.  Are we talking RC or CC here, or do you make a distinction in 
this case?  In a rib crowned and supported board I thought that all the 
load was taken by rib bending.  I thought that the panel didn't enter into 
it.  What would be resisting or supporting this panel compression?  I 
believe you would say it's not the rim.


>>>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 
>spring rate, and the board is already flat, that means the panel is 
>already too crushed to bend the rib into a crown, much less provide the 
>compression resistance to increase spring rate as it is loaded.
>
>
>>> >> But I think it is getting stiffer.
>>>      Dale
>
>Me too.
>
>Ron N

I think I'll wait until after measuring some boards in pianos before I decide.

Phil Ford