evaluating sdbd. crown & bridge downbearings in a new piano

[Frank Weston]

Ron wrote:

>If you are talking about alternate realities, and soundboard assemblies
that
>are built with no intention of producing a working soundboard, I wish you
>would say that up front and save everyone some time. I, at least, was
>attempting to seriously discuss practical concerns and real world
phenomena.
>


Ron,

First, I want to change terminology.  From now on,  a "compression" crowned
soundboard will be referred to as a traditional or standard construction
soundboard.  Any other method of soundboard construction shall be refereed
to as "alternative".

Second, I want to point out that moisture content per se has no effect on
the compression of wood.  What stresses wood is deformation.  This
deformation may come as a result of change in moisture content acting in
concert with some kind of constraint on the wooden assembly, but without
constraint, simply adding or subtracting moisture will not compress wood.

Now, let's build a soundboard assembly.

We use mechanical means to deform a flat panel of spruce to a crowned shape
(a press, a form, a vacuum bag, etc.)  The inside of the curve compresses,
and the outside is stretched -  what will be the top is in tension.  We glue
ribs to this panel.  They are held in place by mechanical means (press, go
bars, vacuum bag, strong fingers, etc.).  The ribs are bent and they are
stressed just as is the soundboard - the side of the rib against the
soundboard is in tension, and the side away is in compression.  When the
glue dries and all constraints to the assembly are removed, the soundboard
has a crown.  We have not changed wood moisture content.  The crown is
formed by the moment due to the difference in tension and compression
between ribs and panel.   The crown side of the soundboard is in tension,
the rib side is in compression.  If mosisture content of the assembly
changes, the top of the soundboard may be in less tension, or even
compression, but the final load depends on the moisture content chosen by
the builder to start and the moisture content at the time of measurement.
Not to put words into Mr. Burkett's mouth, but this is a point I think he
has been trying to make.

One of the most successful and largest volume restorers on the East Coast
uses a method similar to the one I describe above.  They have tried
alternative methods (shaping ribs) but had an inordinate amount of failures
of soundboards so constructed and long ago reverted to traditional
construction.  Their observation was that not only were the alternatively
constructed soundboards more likely to fail, but they were also less
responsive musically.  Steinway uses similar methods to those I described
above, but they do dry their wood down before assembly.  As a result, their
soundboards are likely to have less tension or possibly even compression in
the top half of the panel.

With pianos, the proof is in the pudding.  It is unlikely that given the
extreme competition in the piano industry over the past 150 or so years that
every advantage would not have been explored and exploited for commercial
gain.  The "alternative" rib shaped soundboard is not a new idea.  It is not
a particularly good idea, and to paraphrase Del, no amount of argument is
going to change that fact.

Have a nice day,

Frank Weston