Soundboard stress cracks

[rhohf@idcnet.com]

In over 20 years of repairing (yes, repairing) what seems like miles of
soundboard cracks, I have had ample opportunity to muse on the effects of
stress on soundboards.  I believe that it is possible in many cases to
diagnose normal and abnormal stresses built into a soundboard based on the
types and patterns of cracks that form.  There are two fundamental "rules"
that come into play here:  first, in order to crack, wood must be under
tension (compressed wood can't crack), and, second, that wood expands and
contracts about twice as much in the tangential direction as in the radial
direction.  The latter is described in _Wood Handbook_ published by the Forest
Products Laboratory, Madison, Wis. and (I think) a _Journal_ article a few
years back.  However, there are always individual boards that are unruly for
reasons difficult or impossible to detect.

This twofold expansion means that a soundboard expands much more per unit of
dimension in thickness than in width and would tend to indicate that the
practice of using quarter-sawn lumber in soundboards (and bridge caps) is for
structural rather than acoustical reasons.  Consider what this twofold
expansion means at a glue joint between two (perish the thought)
not-so-quartersawn boards.  Here the differential expansion and contraction is
occurring in planes which are not either parallel  or perpendicular to the
plane of the soundboard.  If these imperfect boards meet at the glue joint in
such a way that the annular rings form an inverted "V", the twofold expansion
can focus compressive stress in a diagonal direction from both sides of the
joint, forcing the wood upward into a "compression ridge".  See for yourselves
if you can observe this type of compression ridge in places where the end
grain is visible.  I am the first to admit that this example doesn't explain
all ridges, and am at a loss to explain why some of these "improper" joints do
not form ridges.

Bob Hohf
Wisconsin