Hi Terry...
Perhaps I should have said "a" primary instead of "the" primary... that
said I think that the way I put it isnt really far from the truth. Once
upon a time ribs were put on flat soundboards that had no appreciable
downbearing to support... or what ? Anyways... in case you havent come
across this bit about anisotropism.... I submit the below.
Cheers
RicB
From Wogram:
"A stiffening of the soundboard would improve the sound radiation
efficiency, as a stiff soundboard is less inclined to subdivide into
small vibrating areas. One purpose of the ribs is precisely to
stiffen the soundboard, which indeed is thin in proportion to its
size. Another purpose is to "homogenize" the soundboard by
equalizing the difference in bending stiffness (elasticity modulus)
parallel to and across the grain. The moduli of elasticity in the
two directions are in a ratio of approximately 20:1 (anisotropism).
If this anisotropism is not compensated for by the addition of ribs
running across the grain, the effective vibrating area is reduced
and the radiation efficiency is decreased over a broad frequency band."
From an online dictionary:
Definition of: anisotropic
"Refers to properties, such as transmission speed, that vary
depending on the direction of measurement."
A good read is
http://www.speech.kth.se/music/5_lectures/wogram/ribbing.html
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