Impedance matching

[Jim Coleman, Sr.]

Hi to all interested:

Although I have no idea about any numbers which could be attached to
impedance of strings and soundboard, my take on this is as follows:

If a soundboard is very compliant with the movement of the strings, there
is rapid transfer of energy from strings to board which results in loud
percussive sound but short duration.

If the soundboard is very stiff and resists the acceptance of the string
energy, we have the opposite effect where the duration is increased greatly.

The ideal tone for a piano is such that there is a good trade-off between
compliant and resistive boards, but this of course is different in different
parts of the board. We still want power, but with a good dose of sustain.
I picked up this idea from Dr. Kent when Chuck Walter and I were working 
under him at the Conn Co. Our first piano design was for a 36" spinet where
we deliberately used a reverse crown but had plenty of downbearing. This
certainly eliminated the possibility of the soundboard developing the
oilcan effect. The second piano we designed was the prototype of the present
Walter piano. In this design, we planned on a crowned soundboard. Our 
thinking at that time was that like an egg which has a great deal of
compressive strength when force is applied evenly; a crowned soundboard has
more compressive strength when the force is applied evenly. The disaster is
possible if the force is concentrated in a local area. Try this first with
a boiled egg. Hold it in your hand and squeeze with even force in your 
hand. You cannot break it. In case I'm wrong, at least you will not get egg
on your face if you take the precaution of using a boiled egg. However, if
after you slice the egg in two and scoop out the good stuff, you will find
it much easier to break the half shell while pushing from the inside than
from pushing on the outside. This is a good illustration of the importance
of crown in soundboards. The soundboard is stronger in compression than in
tension.

Some might suggest that we should just have thicker soundboards if strength
is what we need. However, it is stiffness we want without the mass loading.
The industry has empirically discovered the basic soundboard thinkness
which is needed. They only vary primarily in the sculpturing processes and
in the rib loading.

Jim Coleman, Sr.