More on soundboard crown

[Richard Brekne]

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Delwin D Fandrich wrote:

>
>
>      ----- Original Message -----
>      From: Richard Brekne
>      To: Pianotech
>      Sent: August 18, 2003 10:38 AM
>      Subject: Re: More on soundboard crown
>
>
>
>      Now correct me if I am wrong.. but I understand this to mean
>      that a CC board will be stiffer, both unstrung... and
>      increasingly so when downbearing is applied.  Assuming that
>      is correct,
>
> Which it is not.
>
>

Please refer again to the statement made by Ron

     "Given a panel crowned assembly with a severely dried  panel,
     and one rib crowned with a non dried panel, both to identical
     crowns  at room EMC, the one with the higher panel compression
     level will have the steeper spring rate gradient, and will be
     stiffer."

Under the qualifiers that we are talking about boards with same amounts
of crown at the same RH. If then I've misunderstood the above quote...
then it would be nice to hear what was really meant here.




          and putting aside all other compression damage
          discussion for a second or two... I have a question
          about this long term so called compression set.

     By this are you implying you don't believe the phenomena
     exists? Check The Wood Handbook (USDA) and Understanding Wood
     (R. Bruce Hoadley).




Jimmenees guys.... I was not under the impression that the term "so
called" implied anything at all. Of course I recognize the validity of
the phenomena.



          Given the absolute ideal climatic conditions over
          many years... would a soundboard subjected to
          uniform compression set take on a permanent
          stiffness as a result of that, or is it exactly the
          woods resistance to being compressed that causes the
          stiffness ? Seems to me I also read something in one
          of Johns posts that hinted in this direction. That a
          CC board <<properly treated>> will eventually be
          pressed into a very stiff and permanent condition.

     No. Compression set continues until some neutrality is
     reached. I.e., the point at which there is so little
     compression left within the panel that in practical terms the
     wood fibers no longer deform. At this point compression and
     crown and stiffness will be at their minimum point.

AH !!... Thankyou... That was what I was after. So.... stiffness then is
in a sense a function of how much compression there is in the board, but
if the degree of compression is beyond what the wood can handle it will
loose both ?



          On the side of this... and please forgive whatever
          lack of knowledge I display in the asking,  if
          compression from ribs being glued cross grain to the
          panel is such a problem...even in RB boards.. is
          there no way of contriving a rib such that it is a
          bit less constraining ?

     I don't understand any part of this paragraph:

         -- What compression from the ribs? In a
     compression-crowned soundboard system the compression is in
     the soundboard panel, not the ribs. The rib-crowned system
     uses an entirely different technology that has been described
     in infinite detail numerous times in print and on this list.
         -- Compression from ribs is not a problem.
         -- Ribs, in a rib-crowned system, can be contrived to do
     just about anything you want them to do.
         -- What do you mean by "less constraining?"


Thanks for asking for a clarifier Del. Kind of you. I meant that it is
the ribs constraint upon the panel that cause the panel to either
compress or become tensioned depending on what the MC was when the ribs
were glued on and what it is at any other given time. If RH brings MC
below that starting point the board becomes tensioned right ?? And when
RH causes MC to rise above that starting point there will be some
compression. So assuming that Climatic conditions can get over stress
even an RC panel... I just was musing if there was some way of contiving
a rib that gave crown support, but was a bit less constraining to the
panels tendancy to expand and contract across the grain.... sort of
sliding ribs if you get my meaning...Probably a silly question I know...
but I was just curious.

Del


Thanks for clearing up most of it for me... I am still a bit foggy on
what Ron meant in the first part tho. I suppose he will bring out his
mace and try and smash it into my skull for me :)

Cheers
RicB

--
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html
http://www.hf.uib.no/grieg/personer/cv_RB.html


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