More on soundboard crown

[Mark Davidson]

Some thoughts - more on "why" than "how".

The Yamaha piano salesman pulled out this tuning fork
attached to a piece of wood.  Whacked it so it made noise,
then flexed the wood.  Sound volume increased dramatically.
Gimmicky, but interesting.

Been reading the CC vs RC discussion and had a few thoughts.
My intuition tells me the thing that really matters is the shape,
not the tension.  I'll try to explain why. (Keep in mind I know
squat about soundboards.  Absolutely zero practical experience.)

Think about speaker cones.  Made of thick paper/thin cardboard, yet they
can make a lot of noise (more than many pianos).  What is the key?
The cone shape allows you to put pressure on it (only in one direction)
without deforming it (much).  That means the whole cone moves when you
push in the center.  Allows you to move a lot more air.  This works because
of the particular geometry, lack of elasticity of paper, but not due to any
inherent tension in the material.

I think there is something similar going on with the piano soundboard.
By curving it (is it a section of cylinder or sphere?) the movement of the
board is more constrained.  Wood is quite flexible but not so elastic.
Once curved along one axis, and glued to ribs that maintain the curve,
the whole board wants to move together when pushed in the middle.
Even more so when you glue a bridge along the crown.
I think that's the goal, to get more of the board moving. And
the best shape for meeting that goal is curved (spherical section
would probably be ideal, but hard to make).

But there are a few other competing problems.  The board must 
have adequate pressure against the strings to transfer energy, 
tending to flatten out crown.  This is where the tension comes in.  
There must be enough tension in the board to both push against 
the strings and maintain crown. And must not be too stiff 
around edges of board which would constrain movement.

Warning: bizarre stuff ahead.

So perhaps the ideal board is not fastened around the edges, 
but instead held against the strings by...magnets! <GGGG>  
Magnets along the bottom of the board under the bridge, and
a beam below with opposing magnets on adjustable capstans. 
You could do a pitch raise by adjusting the capstans!
This would reduce tendency of crown to flatten, and reduce 
piano going out of tune when weather changes.  At the
same time frees up the board to move and generate more sound.

-Mark