<!doctype html public "-//W3C//DTD W3 HTML//EN"> <html><head><style type="text/css"></style><title>Re: Soundboard crown</title></head><body> <blockquote type="cite" cite> <blockquote type="cite" cite>       Calin<br>       As I'm sure you know the board your describing to be made with flat ribs will not stay straight but crown once the dried panel gains moisture. What you're describing is the old syle  pure compression type of board making. That is unless you mean that  no panel drying is done prior to the rib glue up. In this hypothetical case some amount of bearing is no doubt likely to reverse the crown of the board don't you think. And the panel will likely crack as soon as it dips below it's realitively high E.M.C at the time of ribbing....</blockquote> <blockquote type="cite" cite>   Regards---Dale<br> </blockquote> </blockquote> <div><br></div> <div>Dale,</div> <div><br></div> <div><b>What</b> makes you think it will crack?  Because the board is in tension from the downbearing and as it gets dryer it goes into further tension?  Do you think that would be enough tension to exceed the tension strength of the board?  I wonder.  I also wonder what causes cracks in boards to occur.  Do you think it's from tension exceeding the nominal allowable strength of the wood?  Or is it from compression damaging a section of the wood during a period of high EMC which then weakens it so that the allowable strength (in tension as well as compression) is reduced, so that the next time it sees a low EMC it cracks.</div> <div><br> I also wonder how a flat board that is pushed into reverse crown by the downbearing will behave with changing humidity.  With decreasing humidity the board wants to shrink.  It seems to me that this would mean that the board would want to move up, back toward the flat position.  But the ribs are resisting this.  So a moment would be set up between the ribs and board that would want to push the board down.  The reverse would be true with increasing humidity.  The board wants to grow, which seems to me would mean moving downward.  But the moments between ribs and board would be tending to push the board up.  So perhaps this configuration is actually more stable than the conventional configuration.<br> <br> With a conventional board having crown and with a downbearing load I think the board is always in compression.  However, with this reverse crown board I can imagine that there is some humidity level where the expansion of the board induces a compression load in the board which just balances the tension load from the downbearing so the board is in an essentially unloaded state.  As Del was conjecturing in another post, the acoustical behavior of a board in this state may be very different from a conventional board.<br> <br> Didn't you post some time ago about a piano (I think it was a Sohmer) that came through your shop that had a reverse crown which you thought was deliberate?  I would think that this board would be in tension.  I have trouble imagining how you would build a board like this to be in compression unless the ribs were on top.  Which side were the ribs on, by the way?  I think I remember you saying that this piano had a very nice sound.  Did it have a lot of cracks?</div> <div><br></div> <div>Phil Ford</div> </body> </html>