<!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 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 text=#000000 = vLink=#551a8b> <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></BODY></HTML>