evaluating sdbd. crown & bridge downbearings in a new piano

[Frank Weston]

Del wrote, and I'm arguing (again):

>This is also true of a rib-crowned soundboard assembly.  It's just
>that with the rib-crowned soundboard assembly the MC of the wood panel when
>bellied is somewhat closer to the the norm encountered in the average home
>or hall.  So the maximim amount of compression stress within the panel will
>be somewhat lower over the life of the piano.

I hate to keep beating this horse, but nothing prevents a conventionally
crowned soundboard from being fabricated at a moisture content close to the
norm also.  For the reasons I have put forth previously, such a soundboard
will actually be under less compression when loaded.


>> When the
>> glue dries and all constraints to the assembly are removed, the
soundboard
>> has a crown.
>
>But, that crown is less than it was before the constraints were removed.
It
>is less than the crown of the press.  That is, the radius of the soundboard
>assembly is larger than the radius of the cauls or press table.
>

We can agree that this is true, but so what?  There is a crown, and the
radius is predictable.

>When the ribbed soundboard assembly is taken out of this curved press,
there
>will be a certain amount of springback.  That is, the radius of the
finished
>panel in its free state will be somewhat larger than it was in the press.
>The ribs will attempt to return to their straight condition.  The
soundboard
>panel will resist this attempt and will begin to compress.  When the two
>reach equalibrium they will be at some radius that will be larger than it
>was when they were glued up.  It is this springback that places the
>soundboard panel under compression.

Del, I don't want to accuse you of voodoo engineering, but as long as the
top of a panel constructed as previously described has a curve, it will be
in tension.  No matter how and by what forces the curve is imposed, the laws
of Newtonian physics decree that the outside of the curve will be in
tension.

> As with all soundboard assemblies --
>regardless of the technology used -- the final radius of the crown will
>still be a function of the MC of the wood before it was ribbed and its MC
>after the ribbing is complete and the whole assembly has reached
>equalibrium.

Absolutely true.

> If there is no difference in the MC before and after bellying,
>the crown radius will be relatively larger.   If there is a larger
>difference in the MC before and after bellying, the crown radius will be
>relatively smaller.

We totally agree.

> The question is not whether or not the panel is under
>compression, rather how much compression is it under.

Maybe we can agree here if you will agree that tension and compression are
both stress, positive and negative.  Make that paragraph read: The question
is not whether or not the panel is under stress, rather how much positive or
negative stress is it under (and where).


>
>The ribs will still have the stresses you describe, however.  That is, the
>surface of the rib glued to the panel will still be under tension, and the
>opposite surface will be under compression.  Keep in mind, though, it takes
>energy to keep the rib in this curved condition, and that energy comes from
>the compression stress within the soundboard panel.

There is compression in the bottom half of the panel.  There is tension in
the top.  How can you admit that the rib top will be in tension, but deny
that the panel top, bent the same way, will not be?

>
>Think about it this way.  The shortest distance between two points is a
>straight line.  It is shorter to through a circle than to go around it.


We agree again, if you stick to Newtonian physics.


>Again, this flies in the face of the experience of the rest of the
industry.
>Increasingly, both piano builders and rebuilders are finding that the rib
>crowned board is more controllable and predictable.

This is an easy statement to make, but a difficult one to support with hard
data.  You must admit that you are a little biased on this issue, and that
your personal preferences may tend to cloud your perspective.  I don't know
of m(any) major piano manufacturers who use rib crowned boards.


>Well, at least we agree that the proof is, indeed, in the pudding.  Or, in
>the performance of the piano.  As time goes on, you will see even more
>manufacturers dropping the practice of crowning soundboards by the
>traditional practice of placing the panel under extreme compression.  This
>transition is taking place despite the incredibly strong forces of
>tradition.  It is taking place because the rib-crowned soundboard
technology
>is proving to have better tone characteristics and because these
soundboards
>are proving to be generally less troublesome when subjected to the varying
>atmospheric conditions that the piano tends to be exposed to.

We can agree again.  Time will tell.  I maintain that time (the last 150
years or so) has already told.  We are not talking rocket science here.
Builders of the past tried all sorts of methods, discarded the ones that did
not work and kept the ones that did.  Your next paragraph supports this
thesis.

>
>By the way, crowning the soundboard by cutting a curve in the ribs is not
>really all that new an idea.  It was already a well established practice by
>the time Wolfenden wrote his book, "The Art of Pianoforte Construction" in
>1916.  He indicates that the practice was well known by the time he wrote
>his book

>So, I guess it mostly depends on which tradition it is that you want to
>embrace.  And on how old you want that tradition to be.


And again, we can agree.  "It depends".

Respectfully submitted,

Frank Weston