Hi Frank. I want to read this a bit more closely before offering any thought to the points and questions you raise. I just wanted to qualify a bit what I mean with the sentence you quote below. To begin with, I don't see any real analogy that perfectly fits. But one thing seemed clear to me nearly immediately after hearing arguements for and against compression reliant soundboards. The panels compression alone is simply not what does the job of crowning. Not even in a purely CC board. The rib tenses... if it didn't it wouldn't bend. Its kind of like a cable that is attached the whole underside of the panel and each mm of cable length is in contact with the panel. But the bit I find significant from the analogy perspective is that as downbearing is applied, the <<cable>> strains against the panel straightening out. The more you exert down bearing the more the panel compresses and strains the rib... perhaps (probably to some small degree) to the point of exerting some absolute tensioning i.e. lowering centroid line very slightly. As relates to your point about the high point of the <<arch>> / thickest part of the rib... I agree... the end to end cable analogy doesn't work there. But the way the ribs strain against the compression strength of the panel is similar none the less... or so it strikes me. Perhaps a cable that is thicker directly under the bridge would do the same thing ? I'm out of my league with that speculation... but I thought I'd throw it out there anyways. Cheers RicB pianoguru at cox.net wrote: > ---- Richard Brekne <ricb at pianostemmer.no> wrote: > >> I still think the whole thing functions more like a cable supported arch >> then anything else. >> > > This brings to mind disagreements that I have had with other piano engineers. Given a rib with a cut radius of 60’, e. g., they would argue that a 60’ radius of an arc segment at one position on a full circle is not the same, when shifted to another position on that circle. I would argue that a 60’ radius is still a 60’ radius no matter where it is positioned on that circle. I came to realize that what they were really trying to say is that the thickest part of the rib must always be directly under the long bridge. I have two problems with this: > > First, with multiple ribs on a soundboard, each with a predetermined radius, and its high point under the long bridge, as the above would argue, the points of intersection of the end points of each rib with the rim would be quite irregular and if taken “literally” would exaggerate the potato chip effect, think Ruffles (Ruffles has ridges). If not taken so literally, and the inner rim is surfaced to a more reasonable shape for supporting the crown, what would have otherwise been ripples still constituted unnecessary irregular internal stresses in the soundboard assembly. > > Secondly, given your “cable supported arch” analogy, you would normally think of a cable supporting the arch as a cable extended from an end point of the arch to the other end point of the arch, which would place the high point or the arch at its center. To shift the high point, (or thickest part of the rib, to a position under the long bridge), would be analogous to moving the cable support of the arch from one end point to something short of that, to shift the high point off-center of the arch. Would not an arch be better supported by a cable between its end points, than from one end point, and something short of the other end point, regardless of where along the arch a force is applied, which would collapse the arch if it were unsupported? Would this analogy hold true for rib? Or, does any of this really even matter? > > Frank Emerson > >
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