Soundboard grain angle

[Ron Nossaman]

>Ron,
>You make a good point here about the very top end of the bridge being near 
>the end of the
>'beam' and the beam being stiff there.  I did a quick check of deflections 
>along the grain
>assuming the 'normal' grain orientation and an orientation 90 degrees to 
>normal using some realistic dimensions for a load applied at the very top 
>end of the bridge.  For a load at this
>point the board is in fact stiffer along the grain with the normal 
>orientation than it would
>be if the orientation were switched by 90 degrees because the load is so 
>close to the edge
>of the board.  However, if you move down the scale a bit, in to the so 
>called killer octave then
>the story is different.  If the grain were oriented at 90 degrees to the 
>'normal' orientation you
>get considerably more stiffness along the grain than you do with the normal 
>orientation.

Phil,
Yes it would, at the expense of the treble where stiffness is needed most.
Consider too that you are also making the area just below the killer octave
stiffer than you might want it as well. That's why I think the elevated
grain angle is a good workable compromise. I've used from 60° to 70° from
the belly bar, and found it to be pretty controllable with a fanned rib
pattern. The assembly can be made anywhere from way too stiff for good
volume, to way too flexible for good sustain (or good anything, for that
matter), anywhere from the start of the last octave on down by just rib
number, dimension, and placement. The designer has the control rather than
the materials and construction configuration. With conventional grain
direction and compression crowning, there is a limit to the stiffness that
can be achieved with any given panel thickness and still get some crown. If
you flip the grain angle 90°, there will be a low limit to flexibility as a
result that may not be low enough to meet the requirements. In both cases,
you're at the mercy of the individual characteristics of the panel,
relinquishing some of the control you would have with the ribs in the more
conventional grain orientation. Consider too, that the bridge doesn't act
like a beam the length of the bridge. It's curved, so the beam-like support
of the bridge is more localized to some segment of the bridge that's
effective length depends on the degree of curvature at that point. That's
one of the reasons the soundboard goes flat in the killer octave first in a
traditionally compression crowned soundboard. Check and see where the
furthest deviation from a straight line from one end of the tenor bridge to
the other falls. So it's not so much whether any configuration is
inherently stiffer or more flexible, it's getting to chose the degree of
stiffness and mass to the broadest controllable degree in any given area of
the scale that is desirable.    

Ron N