Ron Nossaman stated in a previous post that (if I understand correctly) he designs a rib set, for a rib-crowned board assembly, to give a certain set of ranges of deflection under different humidity or dryness conditions, for a certain range of expected downbearing forces from one end of the bridge(s) to the other, and that one of the criteria for the bottom of the ranges of deflection is, that the ribs should be the primary elements that make it certain that the board assembly will not go into flat crown or negative crown over the expected humidity and dryness range, and will make this certain for a reasonable expectation of lifetime for the assembly. Factors that can bring about change over lifetime--and in some designs, begin to do so during, or even before, installation--include various possible compression forces within the elements of the assembly, as these interact with each other, and with the strings and case parts. The reason most commonly given for designing for the prevention of flat or negative crown is, that the coupling of the string to the board assembly is otherwise either likely, or certain, to become insecure. But the prevention of excessive deflection in the board assembly-- under any humidity and dryness conditions--is also the prevention of a certain degree of stiffness of the assembly at the coupling, when the string and board are at rest (zero amplitude). This is, in a sense, independent of the question of the coupling's security. The assembly, as a hardening spring, must give that degree of resistance to the downward vertical transmissive force of the string when set into movement--"independently", so to speak, of its downbearing force at rest--to insure, at one and the same time, the (grand) piano's dynamic range, sustain, and desired tonal character. This idea of a certain "independence of function" is, I take it, what has often led to proposals for a different sort of coupling. But it has also led me to wonder about such innovations as Grotrian's "counter-bridge", underneath the piano, under the bridge. Or: *whatever* Grotrian's own purpose may have been, why not consider having one or more counterbridge as a "garantor of crown", in the same sense that Ron's rib set is, and replacing a certain part of that latter function? Ribs are still needed for the solid spruce planks, but - perhaps - the whole assembly could then be designed without as much of a problem of the effects of compression on the panel sub-assembly, i.e., especially the vector of compression that runs across (perpendicular to) the plank's grain? My assumption in all this is, that, in many present designs, the planks benefit--as acoustical components--from having a longitudinal, end-to-end crown, and that the with the new approach, they could keep this, while otherwise suffering no damage. Of course, I'm far from sure. Would this yield a sound that is too much like an early laminated board? And, what *did* Grotrian have in mind with a "counter-bridge"? Randy Jacob University of Michigan Library --On Friday, July 25, 2003 9:34 AM +0200 Isaac sur Noos <oleg-i@noos.fr> wrote: > > Del, > > I suppose we experiment a language barrier limit there . > > I was made aware of the use of the word "tension" in English as being > slightly different from there. > > The idea is that the board may be constrained while glued in, then a > bit more with the humidity coming back, and that is considered > providing a bit of strings pressure resistance (more fast spring) > > Yes we use the term tension to talk about something stretched tight. > > About the "flat bars" against the crowned bars method , Stephen agree > that we are expecting more from the wood, across grain in that > process, and it is more prone to flatten with compression set. But > still, why do we have boards, that where made in the beginning of the > last century , with a perfectly good crown and an inflexion point > under the "killer zone" that does not go to the point front bearing > suffers ? (while more recent production are often suffering of a less > than impressive board) > > He believe that the way the board is ribbed ,then adapted and glued in > the piano is of much importance, the ribbed panel being kept in it's > "bed" under controlled moisture at any time before the final gluing > (done under controlled moisture too). > > For the slant in the belly, he wonder why so many are taking the > effort to compute that slant (Steinway adopted it after some time, > Fazioli made numerous try, Fenner compute them also...) > May be in the end that is overkill, but, particularly because little > differences in placement produce so larges amount of stress in the > system, it should may be not be under considered . > > Greetings > > > Isaac OLEG > > > > > The inserting under tension of the soundboard seem the only way to > > > have some tension in the panel, in the rib direction > > (across grain) > > > the modulus of elasticity is said to be 3 Kg cm2 vs. 100 Kg cm2 in > > > direction of grain. > > > So if we wish to have some tension in the panel that looks like a > > > convenient method, more than compression crowning. > > > > Do you mean tension as in "a pulling force, or stretching > > something tight?" > > Or compression as in, "pressing together by an applied force?" > > > > Generally soundboard panels are under tension only after > > some period of > > having been subjected to compression-set and then finding > > themselves in a > > very dry environment. > > > > > > > > > > Stephen have forced back some boards with shims on the > > straight side > > > with good results he say, when we see the dimensions > > involved, that > > > looks like a possibility. > > > > The problem with this practice is that it works--but only > > very temporarily. > > The physical structure of the wood we use for piano > > soundboards and ribs is > > simply incapable of resisting the stresses created with > > these techniques. > > Entire theories have been built based on the assumption > > that wood is a > > perfectly rigid and non-compressible material. But it is > > not. Even along > > the grain it is a fairly readily compressible material. > > > > The mechanical characteristic involved here is actually compression > > parallel to grain, not the MOE. This value, for spruce (it > > varies with each > > specific species) is approximately 5,600 psi to 6,000 psi > > (36 000 - 40 000 > > kPa). This is not very much considering the small > > cross-section--approximately 0.25 in2 (160 mm2)--of the typical > > feathered-out rib end. > > > > > > > > > > The panel is said to be shaped with 1 cm more than the > > internal size > > > of the rim, was said. If possible a soundboard that > > should be pinched > > > in the rim should work better from an acoustical point of > > view (a very > > > good joke to future rebuilders !) > > > > A good joke on the builder as well, I think. Make a sketch > > of a grand piano > > rim, sketch in the ribs and the soundboard grain angle and > > then think about > > the stresses involved with the proceedure you describe. > > Compare those > > stresses with the known physical qualities of wood and > > you'll see what I > > mean. > > > > > > > > > > Idem on the belly rail, while some brands have stopped > > the insertion > > > there (Seiler for instance). > > > But I seem to understand that in the actual conception in > > the USA the > > > soundboard assembly is viewed as an auto supported device > > is not it ? > > > > In the end they are all auto-supported devices regardless > > of how they were > > installed. A few years of compression-set insure this. > > > > Del > > > > > > > > _______________________________________________ > pianotech list info: https://www.moypiano.com/resources/#archives
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