<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META content="text/html; charset=iso-8859-1" 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 5.00.2614.3500" 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 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></BODY></HTML> <PRE> _____________________________ David M. Porritt dporritt@swbell.net Meadows School of the Arts Southern Methodist University Dallas, TX 75275 _____________________________</PRE>