---------------------- multipart/alternative attachment Thanks for your interesting reply Frank. As for as commiting to one or the other side.. grin... I remain convinced that either both sides are talking about the same thing using different phenomena, or talking about different things without really being aware of it. It seems clear from Dels post a bit back, that a soundboard panel constructed in such a fashion as to be constricted in its growth / size by the ribbing relative room humidity (at any given time) will be under compression to the degree it is constricted. If it is constricted so much that the top of the panel is also constricted then the top will also be in compression. It also seems likely that (since room humidity varies greatly) a sound board could easily be constructed so that the ribs do not force the top of the panel into compression, or that this is the case only over a certain humidity level. When ever such a board crosses that boarder and the top side is in tension, then much of your reasoning follows well enough through. How realistic (or not) it is to build a soundboard which is in tension on the top side is not for me to say. I am still only an egg in the world of piano design.. yea verily not even a layed egg.. <grin> But my eyes are open and my mind works all this over and one question it raises in this regard is the matter of stiffness in a soundboard. I havent gotten any replies to this aspect of soundboards yet so I am still in the dark as to how it fits in.. but I suspect that the more the board is compressed, the stiffer it becomes, and stiffness (to a point) is a very desirable quality in a soundboard. It also seems clear that a CC board built as Ron and Del draw out, dried out rather in the extreme, that has ribs (straight and square) glued onto it before it has had a chance to absorb any humidity back into it, will curl up like a pussy cat, and indeed become compressed on both the top and the bottom. I think you have admited that this can be the case, but counter more or less by saying thats not how soundboards are made. In your example below, you describe more or less a simply bent board with ribbing glued in such a way as to simply hold that bend. In such a situation it is the panel that would want to straigten back out, and the rib that is resisting this. Ergo your idea of tension in the top of the panel and top of the rib, and compression on the bottom of these two. I suspect that if you took your experiment a step further, and subjected it to a few hours (perhaps a day or two) of like 90 % humidity you would find your marks would indicated that the top of the experimental panel was in compression. It might be fun for you to try that. This is a different construction then what Del and Ron are talking about. Ergo.. Either we are talking about two different things, and havent realized that yet, or we are talking about the same thing useing different language and looking at it from opposite perspectives. Thats my call anyways.. Now... anyone want to clear up this buisness of stiffness in the panel for me ??? Richard Brekne I.C.P.T.G. N.P.T.F. Bergen, Norway Frank Weston wrote: > Richard, You have served as a wonderful goad to this discussion, but > ultimately there comes a time when you must commit to an opinion of > your own. What will it be? In a nutshell, my argument is that a > traditionally crowned soundboard can be constructed such that the top > of the board is in tension. I am not disputing that with drying and > manipulation of humidity that the top can be made to be under less > tension, or even compression. What I am arguing is that the final > stress distribution DEPENDS on starting and ending humidity of all > parts. Further, if the top of a board starts in tension and the > bottom in compression, as it is loaded by downbearing, the > tension/compression load will be decreased. Finally, it is my > opinion, that because of the way it responds to load, a traditionally > crowned soundboard will just sound better (this point is pure opinion, > and I won't argue further). Others have made these same arguments in > the past and have tired of butting their heads against the wall of > pedagogy and sycophancy. I now begin to understand their fatigue. I > will not address a situation in which parts are assembled under > differing and controlled humidity. There are too many variables, and > too many correct answers in such a situation. Stress in the board > will vary, and it will depend. Here is an experiment I conducted in my > shop yesterday. I selected a room dried strip of sitka spruce, made > pencil marks on it 18 inches from the end. I bent the spruce to an > approximate 3' radius and glued a rib to the bottom. When I measured > the top of the spruce, I found that the pencil mark position indicated > the top had stretched approximately 1/16 inch, and the bottom had > compressed about as much. This model indicates to me that the top is > in tension and the bottom in compression. I then placed the model > between constraints at each end and loaded the top. As the load > increased, the line on the spruce showed me that tension in the top > was decreasing. I loaded the model to failure. The rib failed first, > and the failure was the most beautiful textbook indication of a > failure in tension on the top and compression on the bottom (if you > don't know what this kind of failure looks like, buy a box of chalk > and break a few pieces in different ways - the most interesting is > when you break it by twisting). Because of one of Newton's laws, we > might deduce that the soundboard was experiencing equal and opposite > loads when the rib failed. If you care to construct such a model, I'm > sure your results would be similar. If you would like to see > pictures, I will consider putting them up on a photo site. If the > actual physical demonstration of the main point I am arguing does not > convince, then I fear we must promote this discussion to the level of > religion where issues are resolved on the basis of faith and > personality. Frank Weston ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... 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