more heresy

[Delwin D Fandrich]

Stephen Birkett wrote:

> I did misunderstand, but this is interesting. The method you describe is
> precisely the method that was used for harpsichords, and all 18th and 19th
> Century pianos as well. I thought you meant the practice of forcing the
> board into a bowl, using curved ribs, etc. ...is this not the common
> practice today? This is what I meant when I said modern practices are
> destructive. Your "compression crowning" and my "differential wood
> movement" then seem to be exactly the same technique.

Apparently we are referring to the same processes. Both systems are
being used today, as well as combinations of the two. Soundboards
crowned by virtue of curved ribs and which are pressed in curved cauls
are just as musical—we think more so—and show every indication of
exceeding the musical life expectancy of their compression crowned
predecessors. This because they are not subjected to the destructive
compressive forces that are found in the compression crowned boards.


> Don't agree...early piano soundboards were crowned in the same way. Won't
> repeat my earlier comments on 183 year old boards...that not only look
> good for crown, but sound good too.

Again, I must object to the attempt to use the exceptions to prove the
rule. Agree or not, this is still basic wood technology. Those
exceptions don't alter the fact that the traditional method of using
expanding wood fibers to create enough internal compression to crown the
soundboard assembly results in internal forces that consistently exceed
the commonly accepted 1% fiber compression limit. This is the maximum
amount of compression that wood cells are typically able to tolerate
without undergoing permanent deformation, i.e., when compressed beyond
this limit, they crush themselves irreparably. (The 1.0% rule assumes
reasonably well handled wood that is reasonably consistent in its
characteristics. Certain flaws in the wood and/or grain orientation
within the panel will lower this limit still further—one reason why at
least some care must be taken in selecting and processing wood for piano
soundboards. I do agree, though, that modern practice does go somewhat
overboard here.)

A typical piano soundboard panel—if it were unrestrained by ribs—will
expand and/or contract by as much as 1.5% to 2% when exposed to the
normal climate changes found in this country. In other words a spruce
panel 1000 mm wide at 3.5 % to 4.0% MC would grow to approximately 1015
mm to 1020 mm at 13% or 14% MC. When that panel is restrained by ribs, a
rigid piano rim and the down force of the 220 or so strings that prevent
it from bowing upward, it has no where to go, so it compresses. This is
what happens in a compression crowned soundboard. Feel free to do the
experiments yourself. I have already done them and don’t really care to
repeat myself. There will always be exceptions, but the above will be
true for most soundboards in most pianos most of the time.

It is possible to build a soundboard assembly that is not subjected to
this amount of internal compression by starting with a panel that has a
uniform moisture content of approximately 6.0% to 6.5% and using curved
ribs and pressing into curved cauls. This process is far gentler on the
wood, both in fabrication and in service. We have found an additional
benefit—as others have, I’m sure—that by using this process it is far
easier to control the acoustic properties of the finished soundboard
assembly. Not all of the old ways were better ways. Since there is no
acoustic or longevity benefit to be gained by continuing a practice I
know to be inferior, this is one area in which I will cheerfully move on
to a new process that I have found to be much better than the old.

ddf