Soundboard stiffness vs. string coupling

[Ron Nossaman]

>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