[pianotech] Fwd: Mason & Hamlin soundboard model with tuning fork

[David Love]

It depends on how you define "function". Stiffness is also achieved by
downbearing and stressing the panel by compressing the spring.  Really the
issue is controlled impedance or the rate of energy transfer combined with
potential energy achieved by compressing of the soundboard spring (and some
other issues as well, of course).  But building stiffness into the assembly
by virtue of making more massive ribs on a flatter panel (or a more crowned
one for that matter but relying less on downbearing) will not achieve the
same acoustical result as a lighter weight assembly with more crown that is
compressed with downbearing to yield a similar level of stiffness.  I would
say that crown is definitely needed if you prefer the acoustical properties
of a lighter weight soundboard whose desired impedance characteristics are
achieved by compressing the panel with downbearing rather than simply
building heavier ribs, and the acoustical differences will be quite
pronounced.  

With respect to the soundboard model with tuning fork, while it doesn't
necessarily demonstrate the effects of crown, it does demonstrate the
effects of stiffening the panel by virtue of flexing it, in this case, not
unlike what happens when a crowned assembly is put under compression.  The
interesting difference is that in the crowned assembly you add stiffness by
compressing it, in this flat model you add stiffness by flexing it into a
crown.  That flexing changes the impedance characteristics of the piece of
soundboard material and it does so by making it stiffer without adding ribs.


David Love
www.davidlovepianos.com

> Crown, per se, isn't necessary to soundboard function. Crown in a
> compression crowned board as is used by Steinway is an artifact of
> high panel compression bending flat ribs up into a curve when the
> panel rehydrates. The ribs are actually resisting the formation and
> maintenance of crown. Loading this panel with string bearing pushes
> the crown flatter, and makes the assembly stiffer as the panel
> compression level reaches it's physical limit. The stiffness is what
> you're after. Given enough stiffness built into the assembly, crown
> isn't needed. Stiffness can be achieved with crowned ribs, acting as
> support beams, but the ribs need to be considerably stiffer than
> those in a compression crowned board to support string bearing with
> measurable positive crown without high panel compression levels.


Ron N