Soundboard stiffness vs. string coupling

[V T]

Ron and Terry,

A couple of questions:

-  When you talk about string-to-bridge "coupling", do
you mean the physical attachment of the string to the
bridge so that the string doesn't bounce off the
bridge, or are you thinking of the mechanical coupling
factor - a mathematical model representing the portion
of string energy that is coupled off to the soundboard
via the bridge?  (My use of the word "coupling" refers
to the second.)

-  On a piano with vertical hitch pins (like a Baldwin
Accujust), if we set the downbearing to zero on a
single hitch pin (but leave all the other strings as
they were), will there be a significant difference in
tone compared to the same string when it has its
factory adjusted downbearing?

Thanks for the thoughts!

Vladan

>I am starting with the assumption that one intended
>benefit of the downbearing force is the increase in
>board stiffness.  It's pretty obvious that the
>downbearing of a single string isn't significant to
>accomplish much change in stiffness.  The increase in
>stiffness is the result of the composite force
exerted
>by all the strings (however that force is
>distributed).

That's correct. Compressed crown increases the
soundboard stiffness, 
whether the assembly is rib or compression crowned.


>There is however another design parameter that
>matters;  that is the amount of downward pressure
from
>a single string.  This factor is important, because
it
>determines the coupling of the individual resonator
>(string) to the bridge/board.  This coupling
parameter
>is a separate issue from board stiffness.

Not exactly.


>So, we have the following problem:  we have to adjust
>two parameters (board stiffness and string coupling),
>but we do it by adjusting one variable - downbearing
>force.

The offset angle of the strings across the bridge, and
the slant of the 
bridge pins provide adequate coupling whether there is
downbearing or not.


>What if the downbearing required to get the
>right board stiffness isn't the same downbearing that
>results in the best coupling from an individual
string
>to the board?

Typically, we decide what bearing we want where, and
size an appropriate 
number of ribs to provide the support and stiffness
required for our string 
scale at the desired bearing distribution. Coupling
pretty much takes care 
of itself if we built the bridges to provide it.

Ron N


		
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