<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META content="text/html; charset=windows-1252" = http-equiv=Content-Type><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 = Transitional//EN"><!DOCTYPE HTML PUBLIC "-//W3C//DTD W3 = HTML//EN"><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 = Transitional//EN"><!doctype html public "-//w3c//dtd html 4.0 = transitional//en"> <META content="MSHTML 5.00.2314.1000" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY aLink=#ff0000 bgColor=#ffffff link=#0000ee style="BACKGROUND-COLOR: #ffffff; COLOR: #000000; FONT-FAMILY: Times = New Roman; FONT-SIZE: x-small" text=#000000 vLink=#551a8b> <DIV><FONT face=Arial size=3>Dave, </FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial size=3>I'm impressed, You said in 21 words = what it took me 1500 words to say.  BRAVO!</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial size=5><STRONG>"When a dried, constrained SB = panel takes on moisture, it is being inflated like a tire.  It's all under compression"</STRONG></FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial size=3>Halleluiah!</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial size=3>I just couldn't have said it any = better.  Now, much to everyone's delight, I'm going to shutup, sit down, and give it a = rest.  This horse has finally died.  But it was one heck of a ride.</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial size=3>Now lets all have a group hug and get along!</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial size=3>-Brian Henselman, RPT</FONT></DIV> <BLOCKQUOTE style="BORDER-LEFT: #000000 2px solid; MARGIN-LEFT: 5px; MARGIN-RIGHT: = 0px; PADDING-LEFT: 5px"> <DIV><FONT face=Arial size=2><B>-----Original = Message-----</B><BR><B>From: </B>David M. Porritt <<A = href="mailto:dporritt@post.cis.smu.edu">dporritt@post.cis.smu.edu</A>&g= t;<BR><B>To: </B><A href="mailto:pianotech@ptg.org">pianotech@ptg.org</A> <<A = href="mailto:pianotech@ptg.org">pianotech@ptg.org</A>><BR><B>Date: </B>Tuesday, September 28, 1999 6:50 PM<BR><B>Subject: </B>Re: = evaluating sdbd. crown & bridge downbearings in a new = piano<BR><BR></DIV></FONT> <DIV><FONT size=3>When a dried, constrained SB panel takes on = moisture, it is being inflated like a tire.  It's all under compression.  </FONT></DIV> <DIV> </DIV> <DIV><FONT size=3>dave</FONT><BR><FONT face=Arial = size=2>*********** REPLY SEPARATOR ***********<BR><BR>On 9/28/99 at 5:04 PM Brian Henselman wrote:</FONT></DIV> <BLOCKQUOTE style="BORDER-LEFT: #000000 2px solid; MARGIN-LEFT: 5px; = PADDING-LEFT: 5px"> <DIV><FONT size=2>Frank,</FONT></DIV> <DIV><FONT size=2></FONT> </DIV> <DIV><FONT size=2>Nothing is PULLING the top to make it expand = (hence, there is NO tension in either a compression-crowned/straight rib or a = tapered rib/rib-crowned soundboard).  The expansion is coming from = WITHIN the wood cells, because as internal water content increases, internal = forces begin to push outward.</FONT></DIV> <DIV><FONT size=2></FONT> </DIV> <DIV><FONT size=2>When humidity is added to a dried = soundboard, all of the cells, including the top, are GROWING from within and pressing outward.  This is not tension, but compression.  = Why?  The argument here is caused by a mis-understanding of term "compression".</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>Try to bear with me here.  You and I = usually think of compression as the increase of force on a cell from the OUTSIDE = pushing in.  This definition as it pertains to wood is fine as long as = the water content stays constant, and the only variable is the increase = of pressure from the outside.  </FONT></DIV> <DIV><FONT size=2></FONT> </DIV> <DIV><FONT size=2>When internal water content stays constant but = outside pressure is increased, the cell shrinks from the increase of outside = pressure pushing in, while simultaneously raising the = pressure inside too.  This is a very simple example of = compression.  One where the cells are shrinking.  However, increasing the = pressure on the outside is not the only way to create compression.</FONT></DIV> <DIV><FONT size=2></FONT> </DIV> <DIV><FONT size=2>(BTW, when you flex your yardstick, you are = "compressing the wood on the the bottom" while maintaining a constant moisture content.  You are also stretching the outer/top side by placing = it under tension.  However, this doesn't apply to  = manufacturing a soundboard.  </FONT><FONT size=2>Here's why:)</FONT></DIV> <DIV><FONT size=2></FONT> </DIV> <DIV><FONT size=2>Think of wood cells with a small amount of = water in them (i.e. a dried soundboard panel).  If we simply added water = to the inside by raising the surrounding air humidity, the cells would = grow.  Would this growth alone cause compression? No, not necessarily, = because the cells aren't being constrained by any additional outside = force.  (Del, Ron, Brian T, I hope I'm right on this point).</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>Now constrain this growth, by first drying out = the panel, and attaching a rib to the bottom of the soundboard = panel.  Now reintroduce humidity.  This increased humidity cause the = internal water content to increase, thus increasing internal pressure, which then introduces the compression to the system for the first time.  = The lower side (ribbed side) is under compression because the cells are = filling with water, but not growing because the rib won't allow = growth.  But, what about the top side?</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>Aha!  The top IS growing, but from the = inside---> out!  This is COMPRESSION, albeit less than on the bottom side, = but still compression nonetheless.  "But How? The top side is = stretching, right?"  Here's how:</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>Think of "tension" for a moment.  Tension = is caused when an object is forced to stretch from a force or forces PULLING = from the outside.  When you flex your yardstick, you are "pulling" the = outside to form the upper/outer arch.  Same goes to Steve and his canoe = analogy.  These ARE examples of tension.  Yes, the top = of a crowned soundboard has "stretched" because it has grown, but this = growth is NOT from outside forces PULLING it into this shape, hence there is = no tension and there never was.</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>As I stated in my first sentence, NOTHING is = pulling the soundboard to create the arch.  "What about that rib?"... you = might ask.  No ribs, not even tapered ribs, PULL the soundboard into = an arch.  They function as stationary objects that either = follow the arch (as in tapered ribs), or resist the arch (as in straight = ribs).  However, these ribs  neither shrink, nor grow to create the = arch (ok, it's wood... so they do did grow, but only a LITTLE, ok).  = However the arch is caused because all ribs RESIST growth, and limit growth = on the bottom by "holding" it, BUT THEY DOT NOT STRETCH the top.  The = top is simply expanding from the growth associated with the increase in = internal water content.</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>Just because an object "stretched" doesn't mean = it is under tension.  Cells swell/grow as they fill with moisture, = however "tension" pulling outward does not cause this growth.  This = growth is caused from "compression" forces pushing outward from within the cell.</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>Are the cell membranes stretching?  YES, = but this is not TENSION!  Why?  Because the cells will continue = to grow until the water content inside the cell EQUALS the water content = outside of the cell.  If you keep increasing outside humidity levels, what = happens?  Well, throw your panel into water, 100% humidity (and = for the purpose of this ludicrous example let's pretend that your glue hold = the ribs isn't water soluable).</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>Do we still have tension in on the top side of = the soundboard? NO.  Even though the cells have reached full = saturation and have expanded to their maximum natural capacity, the rib is still = not STRETCHING the top "outer" surface.</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>BTW, unlike Steve's Canoe analogy, when = manufacturing a soundboard, NO ONE is grabbing the wood and STRETCHING it.  = I'll say this for the last time:  A soundboard's arch is formed from = increasing pressure from within each cell, not from an outside force PULLING outward.  Hence there is NO TENSION in either a = compression-crowned nor a rib-crowned soundboard.</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>I hope this clarifies things into laymens terms.</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>-Brian Henselman</FONT></DIV> <DIV> </DIV> <DIV><FONT size=2>ps, Please forgive any glaring typos in that I = was typing this while holding a tantruming 2 year old in my lap.  NOW = THAT'S TENSION!!! Heh Heh.</FONT></DIV> <BLOCKQUOTE style="BORDER-LEFT: #000000 2px solid; MARGIN-LEFT: 5px; = MARGIN-RIGHT: 0px; PADDING-LEFT: 5px"> <DIV><FONT face=Arial size=2><B>-----Original = Message-----</B><BR><B>From: </B>Frank Weston <<A = href="mailto:klavier@annap.infi.net">klavier@annap.infi.net</A>><BR>= <B>To: </B><A href="mailto:pianotech@ptg.org">pianotech@ptg.org</A> = <<A = href="mailto:pianotech@ptg.org">pianotech@ptg.org</A>><BR><B>Date: </B>Tuesday, September 28, 1999 12:15 PM<BR><B>Subject: </B>Re: = evaluating sdbd. crown & bridge downbearings in a new = piano<BR><BR></DIV></FONT> <DIV><FONT color=#000000 size=2>Del, </FONT></DIV> <DIV><FONT color=#000000 size=2></FONT> </DIV> <DIV><FONT size=2>You are really causing me to think now.  = Are you saying that the tension in a bent (or wrapped) piece of wood won't = cause it to stretch on the convex side?</FONT></DIV> <DIV><FONT size=2></FONT> </DIV> <DIV><FONT size=2>This idea seems to be contrary to the basic = Law of the Universe that states everything has got to be somewhere.  If = the top side of a bent piece of wood does not stretch and the bottom = shorten, careful geometric analysis demonstrates that as you bend wood, are = changing the amount of wood in the cosmos, and we all know that = only God can make a tree.</FONT></DIV> <DIV><FONT size=2></FONT> </DIV> <DIV><FONT size=2>Frank Weston</FONT></DIV> <BLOCKQUOTE style="BORDER-LEFT: #000000 2px solid; MARGIN-LEFT: 5px; = PADDING-LEFT: 5px"> <BLOCKQUOTE style="BORDER-LEFT: #000000 2px solid; MARGIN-LEFT: 5px; = MARGIN-RIGHT: 0px; PADDING-LEFT: 5px; PADDING-RIGHT: 0px"> <DIV> </DIV> <DIV><FONT size=2>Good grief, man, the top piece did not = "stretch," you wrapped it around a larger circle! </FONT></DIV> <DIV><FONT size=2></FONT> </DIV> <DIV> </DIV></BLOCKQUOTE></BLOCKQUOTE></BLOCKQUOTE><FONT = size=2 Arial></BLOCKQUOTE></FONT><PRE>_____________________________ David M. Porritt dporritt@swbell.net Meadows School of the Arts Southern Methodist University Dallas, TX 75275 _____________________________</PRE></BLOCKQUOTE></BODY></HTML>