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
This PTG archive page provided courtesy of Moy Piano Service, LLC