<!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> <br> <blockquote>----- Original Message -----</blockquote> <blockquote><b>From:</b> <a href="mailto:Erwinspiano@aol.com">Erwinspiano@aol.com</a></blockquote> <blockquote><b>To:</b> <a href="mailto:pianotech@ptg.org">pianotech@ptg.org</a></blockquote> <blockquote><b>Sent:</b> August 16, 2003 9:29 AM</blockquote> <blockquote><b>Subject:</b> Re: Soundboard crown</blockquote> <blockquote><br></blockquote> <blockquote><font face="Times New Roman" color="#000040"><br> >>Phil-- My thought was that any wood will shrink over time especially quartered wood because of it's expansion contraction properties and actually that could take some time. However We've all seen old stwy board with no cracks ,compression ridging, crown or bearing for that matter that make my statement  questionable. Or perhaps whatever little compression the wood had originally has equalized or was never great enough to cause compression set or damage.<br> <font size="-1">  </font> 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?  </font><br> </blockquote> </blockquote> <blockquote type="cite" cite> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">Wood does not shrink on its own--at least not so you would notice--only as a result of changing environmental conditions, i.e., with changes in the MC of the wood. Kept in an temperature/humidity stable environment what would cause it to shrink?</font></blockquote> <div><br> I was thinking that normal seasonal variations in humidity would cause it to shrink.<br> </div> <blockquote type="cite" cite> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">Soundboards crack when the wood is placed in tension and that tension exceeds the tensile strength of the wood and, in a piano soundboard this may not be--probably is not--the same tensile strength the wood started out with....So, if a board of particularly resilient and tough wood is ribbed at a relatively high moisture content and the piano spends much or all of its life in a relatively stable microclimate it will survive nicely for some considerable time.</font></blockquote> <blockquote type="cite" cite><font face="Times New Roman" color="#000040"><br></font> <blockquote><font face="Times New Roman" color="#000040"><br> <font size="-1"> </font><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.</font><br> <font face="Arial"></font></blockquote> </blockquote> <blockquote type="cite" cite><font face="Times New Roman" color="#000040"> </font></blockquote> <blockquote type="cite" cite><font face="Times New Roman" color="#000040">>>> If it's reversed the ribs to my mind would now be acting as  a spring in tension and could behave the way you describe. <br>         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> ... I dunno. It could be</font><br> <font face="Times New Roman" color="#000040"></font></blockquote> <blockquote type="cite" cite><font face="Times New Roman" color="#000040"> </font></blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">I assume you're referring to the (so far, unsubstantiated) phenomena of "oil-canning."</font></blockquote> <div><br></div> <div>I don't think I was.  But I'm not really sure what that is.  Can you explain what you mean by this?</div> <div><br></div> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">I don't see how this is expected to happen. The same stress interface between the ribs and panel that originally created the crown will still try to force the soundboard assembly up into a crowned configuration. It would be more likely to develop increasing reverse crown during the dry times. The tensile strength of wood is not all that great, especially after it has been damaged by compression stress.</font></blockquote> <div><br></div> <div>I wasn't really referring to a compression crowned board.  I think the configuration that I have in mind would be uncrowned or reverse rib crowned, if that makes sense.  I wasn't thinking of a CC board that had collapsed.  I was thinking more of a board that was made deliberately made to have a reverse crown, such as some of the pianos that Dale has seen (assuming they were intended to have reverse crown).  So, take an undried panel, glue on some ribs, and load it up.  It will reverse crown and the board will be in tension I think.  Or machine crown some ribs to give the board a reverse crown when they are glued on.  Load it up and once again I think the board will be in tension.  Now if this board is subjected to a drier environment that that at which is was glued up what will happen?  I was assuming that the board would take on additional tension.  Enough to overload it?  I don't know.  But if not, then when this board is subjected to more humid environments than that at which it was glued up it would probably be less likely to sustain compression damage than a CC board, or even a rib crowned board with positive crown.</div> <div><br></div> <div>I was also speculating about what would happen to this board if it was subjected to a drier or more humid environment.  My thought was that in a drier environment the board would want to move up.  My thought was that the board is describing an arc.  As it dried this arc would have to get shorter, so the board would want to flatten out, which in this case would mean moving up.  You seem to think it would move down.  Why do you think that?</div> <div><br></div> <blockquote type="cite" cite><font face="Times New Roman" color="#000040"> </font><br> <font face="Times New Roman" color="#000040"></font> <blockquote><font face="Times New Roman" color="#000040"><br></font> <blockquote type="cite" cite><font face="Times New Roman" color="#000040"><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.</font><br> <font face="Times New Roman" color="#000040"></font></blockquote> </blockquote> <blockquote><font face="Times New Roman" color="#000040">>>>I agree</font><br> <font face="Times New Roman" color="#000040"></font></blockquote> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">I am uncertain what is meant by "a conventional board." Assuming this refers to a compression-crowned soundboard system, the key words in the above are then "having crown." However, a strong argument can be made that this is no longer--if it ever was--the conventional soundboard system....</font></blockquote> <div><br></div> <div>I wasn't intending to comment on CC vs RC boards here.  By conventional, I meant a board that was intended to to have a positive crown, which includes every piano I've ever seen (but not every piano that Dale's ever seen).  I therefore felt safe in calling this 'conventional'.</div> <div><br></div> <div>I was pondering a what if scenario.  What if you built a board that was intended to have a reverse crown.  How would it behave?  Would it have any advantages over a 'conventional' board that is intended to have positive crown?</div> <div><br></div> <blockquote type="cite" cite><font face="Times New Roman" color="#000040"> </font></blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">As well, it is quite possible to have a string downbearing load without having positive crown.</font></blockquote> <blockquote type="cite" cite><font face="Times New Roman" color="#000040"> </font></blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">Del</font></blockquote> <blockquote type="cite" cite><font face="Times New Roman" color="#000040"> </font></blockquote> <div><br></div> <div>Yes, I know.  But in this case it would be a deliberate design feature rather than an unintended consequence.</div> <div><br></div> <div>Phil Ford</div> <div><br></div> <div><br></div> </body> </html>