>First, I want to change terminology. From now on, a "compression" crowned >soundboard will be referred to as a traditional or standard construction >soundboard. Any other method of soundboard construction shall be refereed >to as "alternative". >Second, I want to point out that moisture content per se has no effect on >the compression of wood. What stresses wood is deformation. This >deformation may come as a result of change in moisture content acting in >concert with some kind of constraint on the wooden assembly, but without >constraint, simply adding or subtracting moisture will not compress wood. * Correct, but no one has claimed that it would. >Now, let's build a soundboard assembly. > >We use mechanical means to deform a flat panel of spruce to a crowned shape >(a press, a form, a vacuum bag, etc.) The inside of the curve compresses, >and the outside is stretched - what will be the top is in tension. We glue >ribs to this panel. They are held in place by mechanical means (press, go >bars, vacuum bag, strong fingers, etc.). The ribs are bent and they are >stressed just as is the soundboard - the side of the rib against the >soundboard is in tension, and the side away is in compression. When the >glue dries and all constraints to the assembly are removed, the soundboard >has a crown. We have not changed wood moisture content. The crown is >formed by the moment due to the difference in tension and compression >between ribs and panel. The crown side of the soundboard is in tension, >the rib side is in compression. If mosisture content of the assembly >changes, the top of the soundboard may be in less tension, or even >compression, but the final load depends on the moisture content chosen by >the builder to start and the moisture content at the time of measurement. * Correct, maybe, in this context. How much spring back occurs when the clamps are removed? Depending on the rib scale and panel thickness, the top of the panel could already be in slight compression immediately after assembly, and without any change in EMC, just from the rib stiffness. How deep a crown is being forced in by the panel backing, before spring back? >One of the most successful and largest volume restorers on the East Coast >uses a method similar to the one I describe above. They have tried >alternative methods (shaping ribs) but had an inordinate amount of failures >of soundboards so constructed and long ago reverted to traditional >construction. * Interesting. What sort of failures? Traditional compression crowned construction includes drying the panel before assembly, and gluing and pressing on flat ribs against either a flat, or dished surface. Usually flat. Their method would more properly be considered alternative, though the results could very well be similar to those achieved by traditional methods. >Their observation was that not only were the alternatively >constructed soundboards more likely to fail, but they were also less >responsive musically. * I don't find either to be the case. I find a rib crowned board much more reliably predictable, controllable, and dependable in crown formation, load deflection, and impedance control. I have rib crowned boards out that have outlasted, and are, by my criteria, musically superior to far too many new, expensive, prestegious name pianos with compression crowned soundboards. I haven't got enough of them out there, nor had them out long enough to establish a baseline degredation schedule, but I have looked at and listened to plenty of younger compression crowned boards that have already failed, and precious few rib crowned ones. > Steinway uses similar methods to those I described >above, but they do dry their wood down before assembly. As a result, their >soundboards are likely to have less tension or possibly even compression in >the top half of the panel. > >With pianos, the proof is in the pudding. It is unlikely that given the >extreme competition in the piano industry over the past 150 or so years that >every advantage would not have been explored and exploited for commercial >gain. The "alternative" rib shaped soundboard is not a new idea. It is not >a particularly good idea, and to paraphrase Del, no amount of argument is >going to change that fact. > * In altogether too many cases, exploitation for commercial gain consisted (and consists) of not wasting money on R&D and re-tooling when the customer will buy the instrument for the name, rather than the sound, anyway. Such an attitude is not terribly conducive to evolutionary improvement of the product. No, rib crowning is not a new idea, but I've satisfied myself by building rib crowned boards, that it's a marvelously good idea. A few major manufacturers who have managed to scrape up a few bucks to support an R&D department seem to think so too, since they are making rib crowned boards. >Have a nice day, > >Frank Weston Thanks, I will. You do the same. Ron N
This PTG archive page provided courtesy of Moy Piano Service, LLC