----- Original Message ----- From: Frank Weston <klavier@annap.infi.net> To: <pianotech@ptg.org> Sent: Friday, September 24, 1999 12:08 PM Subject: Re: evaluating sdbd. crown & bridge downbearings in a new piano > 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. This is, of course, true. But, in the piano soundboard assembly, the wood panel is constrained. The soundboard panel has a set of ribs glued to one side of the panel oriented such that they run across the grain of the panel. These ribs act as a very effective constraint. All soundboards -- except those of laminated construction -- are affected by varying levels of humidity. And they all will continue to be as long as they are made of wood. This is also true of a rib-crowned soundboard assembly. It's just that with the rib-crowned soundboard assembly the MC of the wood panel when bellied is somewhat closer to the the norm encountered in the average home or hall. So the maximim amount of compression stress within the panel will be somewhat lower over the life of the piano. > Now, let's build a soundboard assembly. OK. > 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. So far, so good. > When the > glue dries and all constraints to the assembly are removed, the soundboard > has a crown. But, that crown is less than it was before the constraints were removed. It is less than the crown of the press. That is, the radius of the soundboard assembly is larger than the radius of the cauls or press table. > 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. > Not to put words into Mr. Burkett's mouth, but this is a point I think he > has been trying to make. Call them what you will, these will still be soundboard assemblies in which the crown is formed and maintained by across-grain compression of the wood soundboard panel. The effect -- and stress -- on the wood in these panels will be exactly the same as it would be if the soundboard was bellied flat at a somewhat lower moisture content. Indeed, to get this method to work effectively, the moisture content of the soundboard panel will still have to be relatively low compared to the ambient atmosphere. That, or the radius of the cauls, or press table, will have to be VERY small. When the ribbed soundboard assembly is taken out of this curved press, there will be a certain amount of springback. That is, the radius of the finished panel in its free state will be somewhat larger than it was in the press. The ribs will attempt to return to their straight condition. The soundboard panel will resist this attempt and will begin to compress. When the two reach equalibrium they will be at some radius that will be larger than it was when they were glued up. It is this springback that places the soundboard panel under compression. As with all soundboard assemblies -- regardless of the technology used -- the final radius of the crown will still be a function of the MC of the wood before it was ribbed and its MC after the ribbing is complete and the whole assembly has reached equalibrium. If there is no difference in the MC before and after bellying, the crown radius will be relatively larger. If there is a larger difference in the MC before and after bellying, the crown radius will be relatively smaller. The question is not whether or not the panel is under compression, rather how much compression is it under. The ribs will still have the stresses you describe, however. That is, the surface of the rib glued to the panel will still be under tension, and the opposite surface will be under compression. Keep in mind, though, it takes energy to keep the rib in this curved condition, and that energy comes from the compression stress within the soundboard panel. Think about it this way. The shortest distance between two points is a straight line. It is shorter to through a circle than to go around it. > 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. Not knowing anything about the exact proceedure that they used, I can't comment on why they had an inordinate amount of failures. This experience, however, flies in the face of that of the rest of the industry. Including an increasing number of rebuilders. > Their observation was that not only were the alternatively > constructed soundboards more likely to fail, but they were also less > responsive musically. 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. Again, this flies in the face of the experience of the rest of the industry. Increasingly, both piano builders and rebuilders are finding that the rib crowned board is more controllable and predictable. The acoustics of the assembly can be tailored to respond better to different portions of the scale -- bass, middle, treble, etc. -- and the performance of the instrument can be improved. The acoustics of the properly designed rib crowned soundboard assembly are demonstratably equal to, or better than, those of the compression-crowned (or traditional, if you prefer) soundboard assembly. > 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. Well, at least we agree that the proof is, indeed, in the pudding. Or, in the performance of the piano. As time goes on, you will see even more manufacturers dropping the practice of crowning soundboards by the traditional practice of placing the panel under extreme compression. This transition is taking place despite the incredibly strong forces of tradition. It is taking place because the rib-crowned soundboard technology is proving to have better tone characteristics and because these soundboards are proving to be generally less troublesome when subjected to the varying atmospheric conditions that the piano tends to be exposed to. By the way, crowning the soundboard by cutting a curve in the ribs is not really all that new an idea. It was already a well established practice by the time Wolfenden wrote his book, "The Art of Pianoforte Construction" in 1916. He indicates that the practice was well known by the time he wrote his book. Indeed, it was already a common technique that had been used by piano builders for years. He clearly understood that the soundboard was placed in compression across grain, both by the "traditional" method and, to some lesser extent, by his own preferred method of soundboard construction. He also wrote of the value of making the rib part of the load-carrying system by constructing it in such a way as to help support crown rather than resist it. Wolfenden's preferred method of obtaining crown was to use a combination of drying the panel somewhat less than would be required for true compression-crowned construction -- by my calculation his process would have resulted in the panel having a MC of about 5% to 5.5% compared to 3.8% to 4.0% -- and then gluing curved ribs to the panel using curved cauls or a curved press. Wolfenden recommended a rib crown radius of about 60' (about 18 m). Our own process takes this one step further. We control the MC of our boards at a nominal 6.5%. The radius of the ribs is somewhat less than 18 m and varies depending on where the rib lies on the panel. The radius of the cauls is matched to the radius of the ribs. So, I guess it mostly depends on which tradition it is that you want to embrace. And on how old you want that tradition to be. Regards, Del
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