Delwin D Fandrich wrote: > ----- Original Message ----- > From: Richard Brekne <richardb@c2i.net> > >tho from the point of load bearing it seems clear that the rib crown is > stronger, especially in the >longterm. This however might be a "negligable" if > climatic conditions for the finished piano are kept at >an optimum. > > Well, no. It's not really negligible. Even under "optimum" climate > conditions the compressive stress within a compression-crowned soundboard > panel (whether measured on the top and on the bottom of the panel) generally > exceeds the fiber stress proportional limit for spruce. It must to form and > maintain crown in the ribs. And that compression stress will remain high > until it is dissipated by the permanent deformation of the wood cells and > the panel has lost much of its ability to force crown into the ribs (and > support the string load). By which time, of course, the acoustic damage has > been done. Are we sure it is acoustics damage in the direct sense ? Surely structually, which bears on the acoustic properties.. just thinking a bit.. also.. isnt the manner in which string bearing is figured and applied a factor here. Dont strings "help" the sound board to hold the "crown" in the ribs in a CC board after a fashion ? Otherwise the point of fiber stress is well taken and I have yet to see the "opposing side" confront this aspect other then to brush it off, tho I havent had a chance to read carefully Dr. Birketts last. ?? > > Fortunately, there is a certain amount of latitude here. Soundboard > structure are not always as critical as we sometimes make it out to be. So, > while the performance of partially collapsed soundboard may not be optimum, > it may not be all that bad either. Tone degradation -- as a symptom of > soundboard compression damage -- will show up first as a decrease in sustain > time through the 4th, 5th and 6th octaves. If the sustain time through this > region is adequate (to the owner or pianist) I wouldn't worry about it much. > But, if the owner is complaining about a short, percussive sound through the > upper tenor and lower treble sections, don't be surprised if you are unable > to "voice" the piano into submission. > > Regards, > > Del This last is very interesting indeed, and of direct use to those of us who primarily are field techs and perhaps just begining our adventure into rebuilding. You really are quite good at this explaining business Del... ever think of going into business as pedagog ?? grin. But to my point now... At this point in the discussion, seen from my eyes, we have some very convincing argumentation from both Ron N, and Del F. Especially Del has gone into some basic detail as to the physics involved and has left the declarative mode of discussion behind. These last two posts from Dels side have gone basically unanswered by Frank I think, as his last post was intended for other ears I deem. Be that as it may and despite the "engineers" descriptions of the structual problems with CC boards, with some good back up commentary from Brian Trout amoung others.. we have the fact that Steinway and several other well "established superior" manufactures employ this proceedure for their soundboards, and have for a very long time. One has to consider that if the failure rate is so bad as has been hinted at that these companies would have changed policies a long time ago. On the other hand (being the born skeptic that I am) it is not impossible to imagine resonable alternative explanations for this resistance to change. Flipside again,, engineers have been known to make serious blunders despite their arsenal of formulars, figures, and numbers. A disscussion by Franz Mohr some years back about an episode at Steinway NY concerning the placement of the knuckle comes to mind. So again the skeptic in me says to listen closely, but hold open a nice comfortably sized grain of salt in hand in case its needed later. In the end, I guess history will prove the best judge. Still... Del is very convincing. A few questions come to my mind about the acoustic properties of the two configurations. .... Just got in Dels last so some of these are already addressed. One thing I am curious about tho... If the ribs are to support the crown, rather then resist it as you describe Del, my, as yet unlearned, mind in this regard tells me it would seem likely to assume that the ribs, bearing the brother part of the string load, also will become more significant as part of the sound radiation assembly. Does this affect things like placement of ribs, their height, width, mass.. etc ?? Modal frequencies and resonance nodes ??? Richard Brekne I.C.P.T.G. N.P.T.F. Bergen, Norway
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