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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 18, 2003 1:00 =
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Soundboard =
crown</DIV>
<DIV><BR></DIV>Hi Del
<P>I guess what I was getting at was what range of RH would we need to =
confine
a panel to for it avoid tension caused cracking. Actually, as long as =
this is
up. Maybe a rough comparison for safe operating RH's for both CC and =
RC
boards.
<P>RicB </P></BLOCKQUOTE>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080></FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>I can't give =
you a cookie
recipe, no. There are too many variables. Only the principles and =
generalities
I've written about and taught in the past. Once again:</FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>Within the =
continental U.S.
(the only geographical region I've studied to any extent) the climate is =
such
that within the so-called "average" home the wood moisture content =
averages
approximately 4% and 5% (the high southwest =
desert--eastern
California, Nevada, SE Oregon, SW Idaho, Utah, New Mexico and eastern =
Colorado
and Wyoming--during the summer) to a maximum of 12 % to 13% (the western =
seaboard from southern California to Washington and along the eastern =
seaboard
from southeast Texas along the Gulf of Mexico, across Florida and up the =
coast
during the summer). Heating and air conditioning systems can alter =
either of
these extremes considerably. </FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>For example, I =
once looked
at an <EM><STRONG>Unnamed</STRONG></EM> (to protect the guilty) piano in =
Sparks,
Nevada (just up from Reno) with "a soundboard problem." It was January =
and the
outside temperature was about 10º F. Relative humidity was about 25%. =
The home
was heated with a forced-air gas furnace. There was no moisture control =
of any
kind. Still, the soundboard had popped away from the ribs in kind of an
upside-down "V" and, at the joint, was standing about 10 to 12 mm proud =
of the
surface. I have always wondered about the MC of that board when it was
ribbed.</FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>A =
compression-crowned
soundboard made of excellent spruce and bellied and installed properly, =
then, by
design will have zero crown during the summer throughout the region =
mentioned
above. And it will have little, if any, during the winter in the upper
Midwestern states. Neither, in a perfect world and despite the extremely =
low MC,
will there be any tension on the panel. </FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>During the =
summer, on the
other hand, the soundboard panel will be highly compressed if it resides =
anywhere along either coast. I don't remember the numbers off hand but =
taking a
wood sample through the range between 4% MC and 13% MC will cause it to =
expand
by approximately 1%, give or take some. (To find out how much exactly, =
dry a
sample of spruce down to 4% MC and record the dimension =
perpendicular-to-grain.
Then take it up to 13% and record that dimension across the same span. =
The
difference is how much a typical compression-crowned soundboard panel =
will want
to grow and shrink.) </FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>On paper this =
is within the
strength capability of the average piece of spruce wood. But not all =
spruce is
average. Some is above average, some below. As well, and this is the =
problematic
part, any time the MC is significantly above 4%--enough above to force =
crown
into the system--there is going to be some compression-set going on; the =
rate
and amount being determined by how much above 4% the MC goes, by how =
long it's
up there and by the mechanical characteristics of the wood itself. =
As
compression set alters the shape of the wood fibers the panel physically =
shrinks. This is a cyclical process with each high MC swing causing a =
bit more
fiber deformation than the last. Now as the MC drops low and the wood =
fibers
shrink the panel would also like to shrink. Since the natural state =
of the
wood fibers is now just a bit deformed the panel would like to =
shrink to
some physical size smaller than it was when originally ribbed. =
Unfortunately,
the ribs prevent this and the panel comes under perpendicular-to-grain =
tension.
Alas, the wood fibers have also been made weaker by virtue of their =
being
deformed by compression (i.e., they've been physically crushed) and =
their
tensile strength has been reduced. Cracks readily develop. </FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>This is why it =
is impossible
to give safe range of relative humidity for the compression-crowned =
soundboard
system. </FONT><FONT face="Comic Sans MS" color=#000080>It is the MC =
swing that
is the killer. If, once the piano is built, it is immediately taken to =
an
environment that puts the wood at 13% and held there permanently there =
would
eventually be some moderate compression damage (due to the inevitable
compression-set) but cracks would never develop because the wood would =
never
come under tension. And I suppose if you could keep the MC in the 4% to =
6% range
you'd be pretty safe. There would not be enough compression to cause any =
damage
at all, even over the long term. Of course, the soundboard wouldn't have =
any
crown either, but that's another issue.</FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>I mentioned =
earlier (in
another post) that even pianos with compression crowned soundboards that =
had
been located in the Pacific Northwest frequently exhibited little or no
soundboard damage even after some decades of use. I should have added =
that, in
spite of their pristine appearance, rarely are these still =
acoustically
functional soundboards. Time and compression-set have taken there toll =
even
here. The may look good (and be appropriately rebuilt) but their =
soundboards are
acoustically shot. </FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>(I received =
another phone
call just last week from a disenchanted Steinway owner who had his piano =
rebuilt
just last year by a reputable and competent regional rebuilder who =
declined to
replace the soundboard because "it looked really good." The tone, while =
perhaps
acceptable to some, is not at all what the owner wanted. Sustain is =
still short,
especially through the upper tenor and treble, and the piano is, in his =
words,
"lifeless." The work was well done, the board does, indeed, look good, =
the
action works beautifully, the hammers are nicely voiced. And the money =
is spent
and the owner is not at all a happy camper.)</FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>By contrast, a =
soundboard
panel intended for a rib-crowned soundboard assembly will be ribbed at
approximately 6.5% MC. At least our are. At any moisture content below =
this it
will shrink and be under tension, above this it will expand and be under =
compression. But the amount of tension developed is slight and, well =
within the
strength limits of undamaged wood cells. And the MC swing from 6.5% to =
13% is
considerably less than it is from 4% to 13% and the level of compression =
in the
rib-crowned board at 13% MC will also be less. Hence the rate of =
compression-set
will also be lower. And at either extreme there will still be system =
crown (even
if the panel itself is in tension). And there will continue to be system =
crown
even if cracks should eventually develop.</FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>Now in both =
cases I must
point out that the MC swings mentioned are the extremes. In most homes =
they
won't go this low or this high except, perhaps, under special =
circumstances and
that only occasionally. As well, coating the soundboard with a good =
quality
lacquer or varnish will also retard the passage of water vapor into and =
out of
the soundboard panel. </FONT>
<P dir=ltr><FONT face="Comic Sans MS" color=#000080>With a =
rib-crowned
soundboard system exposure to any of the typical environmental extremes =
found in
the continental US should be tolerable. With a compression-crowned =
soundboard
system it is not the cracks that are the problem. It is the loss of =
stiffness
due to the continual deformation of the wood fibers that is the problem. =
If the
piano is going to be kept in an environment that works to provide the =
soundboard
with crown and stiffness, damage is going to be taking place. The only =
way to
prevent that damage from taking place is to keep the piano in an =
environment
that keeps the board in an uncompressed condition. Beyond this, the best =
we can
do with a solid wood panel is to design the thing to minimize the =
destructive
effects of piano's environment while optimizing its acoustical =
potential.
</FONT>
<P dir=ltr><FONT face="Comic Sans MS" =
color=#000080>Del</FONT></P></BODY></HTML>