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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>
<A title=Richard.Brekne@grieg.uib.no
href="mailto:Richard.Brekne@grieg.uib.no">Richard Brekne</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A =
title=pianotech@ptg.org
href="mailto:pianotech@ptg.org">Pianotech</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> August 29, 2003 1:57 =
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Compression =
Question</DIV>
<DIV><BR></DIV> A few days back you said
<BLOCKQUOTE><I>"If the degree of compression is beyond what the wood =
can
handle--i.e., it is stressed beyond its proportional limit--the wood =
fibers
will crush immediately. If it is compression stressed below its =
proportional
limit compression-set will work to reduce that compression stress =
slowly
over some period of time. In both cases there will be forces at work =
to
reduce compression, in the first situation it simply happens
faster."</I></BLOCKQUOTE>So.. given what you say above, am I to =
understand
that a rib crowned board ribbed when the panel is about 6.5 - 7 % MC =
will not
suffer enough compression set during its lifetime... say 30 - 50 =
years ,
to cause any real compression / compression related problems ? =
</BLOCKQUOTE>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>No. Some =
compression set
will occur any time the MC of the panel goes substantially above the MC =
at which
the panel was ribbed. It is a matter of how much compression set can =
reasonably
be expected to occur. </FONT><FONT face="Comic Sans MS" =
color=#000080>In a
perfect world the amount of compression set will be reduced to a level =
that will
not seriously impair the wood's tensile strength at the lowest MC that =
it will
be reasonably expected to survive. In the real world the =
possibility still
exists that the wood fibers might be damaged to some extent but the =
chances this
happening to an extent that will result in visible damage to the panel =
have been
reduced considerably. In addition, if the soundboard's rib system has =
been
properly designed any cosmetic damage that does appear will be just
that--cosmetic. Crown and stiffness are born by the rib system not =
compression
within the panel. I could deliberately cut slots periodically across the =
surface
of the panel (longitudinally with the grain) through to, but not into, =
the ribs
and it would not affect the acoustic performance of the system. (Except, =
of
course, for some acoustical coupling between the top and bottom =
surfaces.) And,
indeed, this has been done. By Vose, I think. In other words the panel =
is now
functioning almost completely as a radiating surface, not as a =
load-carrying
structural member.</FONT>
<P dir=ltr><FONT face="Comic Sans MS" =
color=#000080>Del</FONT></P></BODY></HTML>