<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; = charset=iso-8859-1"> <META content="MSHTML 6.00.2800.1106" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY bgColor=#ffffff> <DIV><FONT face="Comic Sans MS" color=#000080></FONT> </DIV> <BLOCKQUOTE style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; = BORDER-LEFT: #000080 2px solid; MARGIN-RIGHT: 0px"> <DIV style="FONT: 10pt arial">----- Original Message ----- </DIV> <DIV style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: = black"><B>From:</B> <A title=Erwinspiano@aol.com href="mailto:Erwinspiano@aol.com">Erwinspiano@aol.com</A> </DIV> <DIV style="FONT: 10pt arial"><B>To:</B> <A = title=pianotech@ptg.org href="mailto:pianotech@ptg.org">pianotech@ptg.org</A> </DIV> <DIV style="FONT: 10pt arial"><B>Sent:</B> August 16, 2003 9:29 = AM</DIV> <DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Soundboard = crown</DIV> <DIV><FONT face="Comic Sans MS" color=#000080></FONT><FONT face="Comic Sans MS" color=#000080></FONT><BR></DIV><FONT face=arial,helvetica><FONT lang=0 style="BACKGROUND-COLOR: = #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF"> <DIV><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><FONT lang=0 style="BACKGROUND-COLOR: = #ffffff" face="Times New Roman" color=#000040 size=2 FAMILY="SERIF">  </FONT><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" = color=#000040 size=3 FAMILY="SERIF"> Do you think that would be enough tension to exceed = the tension strength of </FONT><FONT lang=0 style="BACKGROUND-COLOR: = #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF">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></DIV><FONT face="Times New Roman" color=#000040></FONT></FONT></BLOCKQUOTE> <DIV><FONT face=arial,helvetica><FONT lang=0 = style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT = face="Comic Sans MS" color=#000080></FONT></FONT></FONT> </DIV> <DIV><FONT face=arial,helvetica><FONT lang=0 = style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF"><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></FONT></FONT></DIV> <DIV><FONT face=arial,helvetica><FONT lang=0 = style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT = face="Comic Sans MS" color=#000080></FONT></FONT></FONT> </DIV> <DIV><FONT face=arial,helvetica><FONT lang=0 = style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF"><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. This condition occurs when the environment around = the piano is quite dry drawing moisture from the soundboard panel causing it = to shrink. Initially, a compression-crowned soundboard panel is installed = at such a low MC that it is nearly inconceivable how it could ever come under = tension. But, over time compression set modifies the wood cells, actually causing = them to deform. With each humid environmental cycle the amount of = compression-set increases. Eventually, the modified--i.e., compressed--condition, or shape, of the wood cells becomes the norm. This is now the = soundboard panel's new equilibrium condition. At this stage whatever crown the = soundboard assembly may have had has essentially dissipated. There is no longer the = requisite compression to maintain the stress interface between the = soundboard panel and the ribs to force crown into the system. At this stage there = will probably also be at least some visible evidence of compression damage on = the panel's surface (though it may not be recognized as such). </FONT></FONT></FONT></DIV> <DIV><FONT face=arial,helvetica><FONT lang=0 = style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT = face="Comic Sans MS" color=#000080></FONT></FONT></FONT> </DIV> <DIV><FONT face=arial,helvetica><FONT lang=0 = style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT = face="Comic Sans MS" color=#000080>This is a gradual and = unpredictable cycle, generally occurring over a period of years. With each year's humid = season the maximum level of compression within the panel is reduced as = compression-set takes its toll. With each year's dry season the level of compression = within the panel decreases, eventually reaching neutrality and then coming under = tension. By this time the wood fibers have become damaged to such an extent that = the tensile strength of the wood panel has also been reduced. As each = weather cycle comes and goes the potential for cracks increases. Depending on the = specific wood used this can take one or two years, ten or twenty years or however = many years.</FONT></FONT></FONT></DIV> <DIV><FONT face=arial,helvetica><FONT lang=0 = style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT = face="Comic Sans MS" color=#000080></FONT></FONT></FONT> </DIV> <DIV><FONT face=arial,helvetica><FONT lang=0 = style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT = face="Comic Sans MS" color=#000080>Wood is a variable material. = Compression set occurs primarily in the earlywood portion of the growth ring and no one = (to my knowledge) has figured out how to predict its effect (i.e., the amount = of compression set, not whether or not it will occur) in any given piece of = wood by examining it visually other than to be reasonably certain it will occur = first in the widest rings.</FONT></FONT></FONT></DIV> <DIV><FONT face=arial,helvetica><FONT lang=0 = style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT = face="Comic Sans MS" color=#000080></FONT></FONT></FONT> </DIV> <DIV><FONT face=arial,helvetica><FONT lang=0 = style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT = face="Comic Sans MS" color=#000080>Why some soundboards never seem = develop cracks is only a partial mystery. Pianos that originally received = rib-crowned soundboards have less tendency to develop cracks. But even pianos that = were originally "rib-crowned" may have been glued up at very low wood MC's. = Wolfenden (in "The Art of Pianoforte Construction")described essentially a = rib-crowning process but still wanted his boards quite dry at the time of ribbing--a combination. Still</FONT><FONT face="Comic Sans MS" = color=#000080>, if these boards do crack the crack is generally not as acoustically = significant. We will even run across the occasional pure compression-crowned soundboard = that seems to have resisted cracking over many years. We can only guess, but = this is probably a combination of exceptional wood, a somewhat wet board at = glue-up (no, their process controls were not perfect), and/or a relatively stable = environment after manufacture. For example, we find less visible soundboard damage = on pianos that have been located in Portland, Oregon throughout their entire = history than on those that have come out from New York or Michigan. 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></FONT></FONT><FONT face=arial,helvetica><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" = color=#000040 size=3 FAMILY="SERIF"></DIV> <BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px"> <DIV><FONT face="Comic Sans MS" color=#000080></FONT><FONT face="Comic Sans MS" color=#000080></FONT><FONT face="Comic Sans = MS" color=#000080></FONT><FONT face="Comic Sans MS" = color=#000080></FONT><FONT face="Comic Sans MS" color=#000080></FONT><FONT face="Comic Sans = MS" color=#000080></FONT><FONT face="Comic Sans MS" = color=#000080></FONT><FONT face="Comic Sans MS" color=#000080></FONT><FONT face="Comic Sans = MS" color=#000080></FONT><FONT face="Comic Sans MS" color=#000080></FONT><BR></FONT><FONT lang=0 = style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=2 = FAMILY="SERIF">  </FONT><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" = color=#000040 size=3 FAMILY="SERIF"><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></DIV></BLOCKQUOTE> <DIV><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times = New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT face="Comic Sans MS" color=#000080></FONT> </DIV></FONT><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" = color=#000040 size=3 FAMILY="SERIF"> <DIV style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; = BORDER-LEFT: #000080 2px solid; MARGIN-RIGHT: 0px"></FONT><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" = color=#000040 size=3 FAMILY="SERIF">>>> 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 </FONT><FONT = lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" = color=#000040 size=3 FAMILY="SERIF">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.</FONT><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 = FAMILY="SERIF"><BR><BR></FONT><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" = color=#000040 size=3 FAMILY="SERIF">... I dunno. It could be</FONT><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" = color=#000040 size=3 FAMILY="SERIF"><BR></DIV> <DIV></FONT><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 = FAMILY="SERIF"></FONT> </DIV> <DIV><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times = New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT face="Comic Sans MS" = color=#000080>I assume you're referring to the (so far, unsubstantiated) phenomena of "oil-canning." 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></FONT></DIV> <DIV><FONT face="Comic Sans MS" color=#000080></FONT> </DIV> <BLOCKQUOTE style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; = BORDER-LEFT: #000080 2px solid; MARGIN-RIGHT: 0px"><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" = color=#000040 size=3 FAMILY="SERIF"> <DIV><FONT face="Comic Sans MS" color=#000080></FONT><FONT face="Comic Sans MS" color=#000080></FONT><FONT face="Comic Sans = MS" color=#000080></FONT><BR></DIV> <BLOCKQUOTE style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #0000ff 2px = solid; MARGIN-RIGHT: 0px" TYPE="CITE"> <DIV><FONT face="Comic Sans MS" color=#000080></FONT><FONT face="Comic Sans MS" color=#000080></FONT><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.</DIV></BLOCKQUOTE> <DIV></FONT><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times New Roman" color=#000040 size=3 = FAMILY="SERIF">>>>I agree</FONT></DIV></BLOCKQUOTE> <DIV><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times = New Roman" color=#000040 size=3 FAMILY="SERIF"><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. There are a = lot more pianos out there with hybrid crowning systems--if not pure rib-crowned systems--than I think we realize. The hybrid systems were certainly = thriving during the early 1900s. True, most of the soundboards found in pianos in = which we regularly replace soundboards were all or mostly compression-crowned, = but this does not necessarily make them "conventional." </FONT></FONT></DIV> <DIV><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times = New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT face="Comic Sans MS" color=#000080></FONT></FONT> </DIV> <DIV><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times = New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT face="Comic Sans MS" = color=#000080>As well, it is quite possible to have a string downbearing load without = having positive crown. </FONT></FONT></DIV> <DIV><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times = New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT face="Comic Sans MS" color=#000080></FONT></FONT> </DIV> <DIV><FONT lang=0 style="BACKGROUND-COLOR: #ffffff" face="Times = New Roman" color=#000040 size=3 FAMILY="SERIF"><FONT face="Comic Sans MS" color=#000080>Del</FONT></FONT></DIV> <DIV><FONT face="Comic Sans MS" color=#000080></FONT> </DIV></FONT></BODY></HTML>