Crown without soundboards

[Jim Coleman, Sr.]

Hi Del:

Thanks for the instruction.

Jim C. Sr.

On Thu, 1 Jan 1998, Delwin D Fandrich wrote:

> 
> 
> Jim Coleman, Sr. wrote:
> 
> > Hi Ed:
> >
> > When one considers a piano soundboard with many splits and there is
> > still downbearing, one might think that indeed the ribs are supporting
> > the downward force of the string bearing. However, when one also
> > considers the fact that as humidity increases, the soundboard rises and
> > increases the tension on the strings, one might think that the board is
> > the crown supporting member. It is well known that the ribs do not
> > lengthen any to speak of when the humidity increases, but the soundboard
> > certainly does swell and rise with humidity increases. Perhaps we can
> > gain a little more insight from the harpsichord builders who have to
> > work with much fewer ribs and funny wavey things that go on with the
> > soundboards.
> >
> > Jim Coleman, Sr.
> > ---------------------------------------------------------
> 
> Jim,
> 
> It is with some caution that one goes about instructing Jim, Sr. But, sometimes possessing more courage than sense, here
> goes...
> 
> Crown in a piano soundboard comes from several sources. At least it can. If the ribs were crowned adequately before bellying
> then most of the crown can, indeed, be supported by the ribs. But, this does not mean that the expanding -- sometimes
> contracting -- panel does not have some effect on the crown radius of the assembly.
> 
> Take an extreme situation as an example. Let's assume that a soundboard was designed an built using ribs crowned to a 60'
> radius. Let's further assume that the moisture content of the soundboard panel was 8.0% when it was bellied. And that there
> is no string load. As long as the moisture content of this panel is held to 8.0% the crown of the assembly will remain at
> 60'. If the moisture content is lowered to, say 6.0%, the crown radius will change. It will become greater -- perhaps 70' or
> 80'. As the panel looses moisture, it will attempt to shrink. The stress differential will actually pull some of the crown
> out of the system. If the moisture content is raised to 12.0%, the crown radius will decrease. Say to 40' or 50'. This will
> be true even if there are numerous cracks in the board, though the effect will be considerably less than it would be if the
> wood were undamaged by previous periods of high compression. You could cut periodic grooves parallel to the grain of the
> panel, and changes in moisture content would still have some effect on the crown radius of the panel.
> 
> Clear as mud, right?
> 
> Del
> 
> 
>