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
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