At 10:25 -0500 13/2/08, Erwinspiano at aol.com wrote:
[ In a message with Subject: (no subject) ]
>JD
> I of all folks highly respect your opinion & experience Êbut what
>it sounded like you were saying I took from you quotes
Yes, I know you don't regard me as an idiot, but I also know your
rapid reading skills sometimes let you down! :-)
>you wrote
>>I take the soundboard out of a piano, take off the bars...the board
>>will be as floppy after the moisture uptake as before. How could I
>>expect it to be stiffer or more flaccid?!
>
> True without the ribs/bars
That's why I said "take off the bars" :-) My posting was in response
to Ric's quotes from researchers into wood, which I was saying were
really not very significant for the range of values for moisture
content in a working piano. If anything the phenomena he describes
will _reduce_ the inherent stiffness of the wood as it takes on
moisture (a problem, incidentally, that is noticeable with hammers)
and _increases_ the compression and hence the potential crown. My
test will be to measure the _inherent_ stiffness of the wood over a
realistic range of values for moisture content. The results will be
interesting, but almost certainly insignificant, because what
interests us is the resilience of the soundboard assembly as a whole.
>>The lack of stiffness of the assembly in the direction across the
>>grain of the board is supplied by the bars, by whichever
>>construction method is used.
>
> Ok this is what I took issue with. Not completely by the bars.
>It's the bars plus the wood under compression.
Yes and no. It is true that once the bars are glued to the board we
are dealing with a single thing that behaves as a unit. In fact the
compressive forces of the bars provide most of the resistance to the
downbearing of the strings because the bars, being compressed along
the grain, are far less compressible and the forces are spent
relatively little in deformation.
> You agreed with me that the stiffness can also be caused by wood
>cell compression yet it seemed like you were saying a few M.c. %
>points didn't make that much difference.. This is what forms the
>Compression crowned boards we all speak of.
Here again, if you read what I wrote, I was talking of the effect of
moisture content on the _intrinsic_ stiffness of the wood, not of the
soundboard assembly. None of us, I am sure, and least of all me,
doubts the magnitude of the forces generated by the uptake of
moisture in wood.
Anyway, enough of all that, and let's come to the question of these
forces without, to start with, getting mathematical and frightening
the horses. In the course of these discussions, the degree of crown
in the finished piano has been touched upon, with on the one hand
Terry Farrell talking of a very marked initial crown and a pretty
substantial final crown on the board-- "I don't get my soundboards
anywhere near flat after stringing", and on the other hand David Love
talking of a less marked initial crown and very little final crown --
"That's pretty much flattening it and there isn't really a problem
with that." and me pretty much in agreement with David Love except
that I'd go further and suggest that near flatness in the strung
piano is probably the ideal because with a flat soundboard the
variable forces generated by changes in humidity will be least likely
to result in movement, and hence changes in pitch.
I am tempted to go on and give interesting examples supporting this
hypothesis but too long postings tend to get left unread. And there
is also a lot more to be said regarding initial compression loading,
range of compression, elastic limits and the avoidance of extremes in
achieving durability and stability.
JD
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