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----- Original Message -----=20
From: Richard Brekne=20
To: pianotech@ptg.org=20
Sent: June 15, 2001 1:37 PM
Subject: Re: Soundboard Evaluation
"If there is still enough structural integrity left in the original =
panel to hold itself together I see no reason why this won't work. And =
work just about as well as a panel made out of new wood. The age of the =
wood is purely incidental. The only thing of any real consequence would =
be the fiber compression that has taken place over the years."
Then I begin wondering where we are going with this point about the =
"consequences of fiber compression".... Follow me for a bit and you may =
see my problem.... Given three rib crowned panels, one of new wood, and =
one of old compression damaged wood, and an old panel that was rib =
crowned origionally. If fiber compression damage is of consequence with =
respect to acoustic performance then the board that was made from =
compression damaged wood should clearly be inferior sounding.... right =
?...=20
I'm not sure. First you say, "Given three rib-crowned panels..." Then =
you say, "If fiber compression damage is of consequence with respect to =
acoustic performance then the board that was made from compression =
damaged wood should clearly be inferior sounding... right?" Which one =
are you referring to? And why are you assuming that any of the three =
should "clearly be inferior sounding...?" How old is that third example =
of your? It might still be sounding great.
and if then all three sound very much the same...then doesnt it follow =
that this fiber compression damage is then, in itself, of no real =
consequence at all in this perspective. ? Do you see my quandry with =
this ?=20
No, I don't. Read again what I said: "The only thing of any real =
consequence would be the fiber compression that has taken place over the =
years." I didn't say that this had any appreciable effect on it's =
performance as part of a working rib-crowned soundboard assembly. The =
compression damage now exists. When the panel was new, it didn't. That's =
all.=20
I repeat, this fiber compression damage will probably have little =
effect, if any, on the panels performance as a diaphragm in a =
rib-crowned soundboard assembly. Just the fact that the fiber damage is =
there shouldn't really matter all that much.
You're just confusing yourself with what is really a fairly simple =
concept and you seem to be trying to blend the two systems together. =
Can't do that.=20
The rib-crowned soundboard assembly does not depend on the across-grain =
compression strength of the wood panel to either form or maintain crown. =
It is simply a wave-carrying diaphragm. The compression-crowned =
soundboard does depend on this across-grain compression strength. If you =
take the ribs off of a compression-crowned soundboard assembly and =
replace them with a set of properly crowned ribs you no longer have a =
compression-crowned soundboard assembly. You now have a rib-crowned =
soundboard assembly.
This is a fundamental difference you must understand if you are be able =
to knowledgably discuss the two. =20
You still seem to be assuming that the wood panel in the rib-crowned =
soundboard assembly you describe (#2, above) somehow depends on its =
being compressed to perform properly as a piano soundboard. It does not. =
It is now simply a diaphragm that happens to be made of a material that =
is anisotropic in nature. The wood panel made of somewhat damaged wood =
started out that way and now, having been damaged somewhat by being held =
under compression for a few decades, it is just a bit more so. As it =
turns out with a rib-crowned soundboard assembly this does not matter =
all that much. Soundboards systems that are primarily rib-crowned do not =
depend on the compression of the wood across-grain to either form or =
maintain crown.=20
The fact that the specific wood used to make this rib-crowned soundboard =
panel--the wood taken from that old compression-crowned soundboard =
assembly--now has less compression strength than it had when it was =
fresh, new wood simply doesn't matter all that much. We are not going to =
use the across-grain compression strength characteristic to structurally =
stiffen the soundboard assembly.
Your question, "If fiber compression damage is of consequence with =
respect to acoustic performance then the board that was made from =
compression damaged wood should clearly be inferior sounding...right?" =
shows that you still have a very fundamental misunderstanding about the =
differences between a compression-crowned and a rib-crowned soundboard =
assembly. This fiber compression damage would only be of consequence if =
I were going to use this panel to form another compression-crowned =
soundboard assembly. In this case it would indeed be inferior sounding =
simply because no matter how much I dried out the old panel I probably =
couldn't get enough expansion to create the stress interface between it =
and the ribs to form the required crown.=20
There is no quandry or magic smoke here. There are simply basic =
differences between two quite different systems to be understood.=20
Ok to take things a step further then... if it turns out that fiber =
compression damage then is of little or no consequence to acoustic =
performance of said panel... then isnt this a rather large point removed =
in the argumentation against compression crowning to begin with ?=20
There is a structural difference between compression-crowned soundboard =
assemblies and rib-crowned soundboard assemblies.
How do you suppose the wood panel acquired that compression damage in =
the first place? As has been explained before, the compression damage =
got there because during manufacture the soundboard panel was made very =
dry (it was shrunk) prior to ribbing with the result that it later =
became heavily compressed (something between 1% and 2.5%) to form the =
stress interface with the ribs that formed the initial crown. Without =
that compression there would have been no compression damage. Even then, =
it is not the compression damage per se that causes the loss of tone =
performance, it is the loss of crown in the soundboard assembly (the =
loss of stiffness) that results from that compression damage that causes =
the loss of tone performance.=20
In other words, the loss tone performance is the result of the loss of =
crown that is the result of the compression damage that is the result of =
compressing the wood to form the crown in the first place.
(we arent into the structural integrity / strength of the panel =
assembly question just yet ok ?... ..cuz I will conceed that point no =
problem anyways..I first want to get at how this fiber damage eventually =
affect the performance capabilities of the wood isolated from all other =
factors.)=20
What?
Ok... so then we would have to confront the strength of these panels =
over time..and if you dont mind... can we take this one step at a =
time... dealing with the above first and as isolated as possible... and =
get into the structual strength issue in the next episode ?=20
Sure. Just keep it civil. Don't assume I'm blowing some kind of magical =
smoke just because you don't understand something.
Del
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