---------------------- multipart/alternative attachment Overs Pianos wrote: > Well put Del. I do hope the doubting Thomas' of the RC school take it > on board. Ron O. I aggree... Well put and nicely answered in all respects. But for what its worth I continue to doubt, as I believe its a healthy thing to do. I am not armed with enough knowledge to judge one way or the other... so instead of just swallowing one sides story hook line and sinker... I keep asking and looking for answers to things that bother me. I find it very difficult to get past the fact that so many of the most successfull manufactures still use CC boards... and indeed swear by them. I also would like to hear more from those advocates as to why. And I simply find the arguementation that goes along the lines of tradition, marketing.. and the rest to be inadequate to explain their stance. So in lue of enough information to make a satisfactory judjement myself... I ask questions...and I remain in doubt. I am really sorry if that bugs a couple of you... but there isnt much I can do about that. Del, again.. thanks for taking the time and patience answer and engage one and all. > > > >> Hi Del >> >> I guess what I was getting at was what range of RH would >> we need to confine a panel to for it avoid tension caused >> cracking. Actually, as long as this is up. Maybe a rough >> comparison for safe operating RH's for both CC and RC >> boards. >> >> RicB >> > > >> At 12:31 PM -0700 18/8/03, Delwin D Fandrich wrote: > >> I can't give you a cookie recipe, no. There are too many variables. >> Only the principles and generalities I've written about and taught >> in the past. Once again: > >> Within the continental U.S. (the only geographical region I've >> studied to any extent) the climate is such that within the so-called >> "average" home the wood moisture content averages approximately 4% >> and 5% (the high southwest desert--eastern California, Nevada, SE >> Oregon, SW Idaho, Utah, New Mexico and eastern Colorado and >> Wyoming--during the summer) to a maximum of 12 % to 13% (the western >> seaboard from southern California to Washington and along the >> eastern seaboard from southeast Texas along the Gulf of Mexico, >> across Florida and up the coast during the summer). Heating and air >> conditioning systems can alter either of these extremes >> considerably. > >> For example, I once looked at an Unnamed (to protect the guilty) >> piano in Sparks, Nevada (just up from Reno) with "a soundboard >> problem." It was January and the outside temperature was about 10¼ >> F. Relative humidity was about 25%. The home was heated with a >> forced-air gas furnace. There was no moisture control of any kind. >> Still, the soundboard had popped away from the ribs in kind of an >> upside-down "V" and, at the joint, was standing about 10 to 12 mm >> proud of the surface. I have always wondered about the MC of that >> board when it was ribbed. > >> A compression-crowned soundboard made of excellent spruce and >> bellied and installed properly, then, by design will have zero crown >> during the summer throughout the region mentioned above. And it will >> have little, if any, during the winter in the upper Midwestern >> states. Neither, in a perfect world and despite the extremely low >> MC, will there be any tension on the panel. > >> During the summer, on the other hand, the soundboard panel will be >> highly compressed if it resides anywhere along either coast. I don't >> remember the numbers off hand but taking a wood sample through the >> range between 4% MC and 13% MC will cause it to expand by >> approximately 1%, give or take some. (To find out how much exactly, >> dry a sample of spruce down to 4% MC and record the dimension >> perpendicular-to-grain. Then take it up to 13% and record that >> dimension across the same span. The difference is how much a typical >> compression-crowned soundboard panel will want to grow and shrink.) > >> > >> On paper this is within the strength capability of the average piece >> of spruce wood. But not all spruce is average. Some is above >> average, some below. As well, and this is the problematic part, any >> time the MC is significantly above 4%--enough above to force crown >> into the system--there is going to be some compression-set going on; >> the rate and amount being determined by how much above 4% the MC >> goes, by how long it's up there and by the mechanical >> characteristics of the wood itself. As compression set alters the >> shape of the wood fibers the panel physically shrinks. This is a >> cyclical process with each high MC swing causing a bit more fiber >> deformation than the last. Now as the MC drops low and the wood >> fibers shrink the panel would also like to shrink. Since the natural >> state of the wood fibers is now just a bit deformed the panel would >> like to shrink to some physical size smaller than it was when >> originally ribbed. Unfortunately, the ribs prevent this and the >> panel comes under perpendicular-to-grain tension. Alas, the wood >> fibers have also been made weaker by virtue of their being deformed >> by compression (i.e., they've been physically crushed) and their >> tensile strength has been reduced. Cracks readily develop. > >> > >> This is why it is impossible to give safe range of relative humidity >> for the compression-crowned soundboard system. It is the MC swing >> that is the killer. If, once the piano is built, it is immediately >> taken to an environment that puts the wood at 13% and held there >> permanently there would eventually be some moderate compression >> damage (due to the inevitable compression-set) but cracks would >> never develop because the wood would never come under tension. And I >> suppose if you could keep the MC in the 4% to 6% range you'd be >> pretty safe. There would not be enough compression to cause any >> damage at all, even over the long term. Of course, the soundboard >> wouldn't have any crown either, but that's another issue. > >> I mentioned earlier (in another post) that even pianos with >> compression crowned soundboards that had been located in the Pacific >> Northwest frequently exhibited little or no soundboard damage even >> after some decades of use. I should have added that, in spite of >> their pristine appearance, rarely are these still acoustically >> functional soundboards. Time and compression-set have taken there >> toll even here. The may look good (and be appropriately rebuilt) but >> their soundboards are acoustically shot. > >> (I received another phone call just last week from a disenchanted >> Steinway owner who had his piano rebuilt just last year by a >> reputable and competent regional rebuilder who declined to replace >> the soundboard because "it looked really good." The tone, while >> perhaps acceptable to some, is not at all what the owner wanted. >> Sustain is still short, especially through the upper tenor and >> treble, and the piano is, in his words, "lifeless." The work was >> well done, the board does, indeed, look good, the action works >> beautifully, the hammers are nicely voiced. And the money is spent >> and the owner is not at all a happy camper.) > >> By contrast, a soundboard panel intended for a rib-crowned >> soundboard assembly will be ribbed at approximately 6.5% MC. At >> least our are. At any moisture content below this it will shrink and >> be under tension, above this it will expand and be under >> compression. But the amount of tension developed is slight and, well >> within the strength limits of undamaged wood cells. And the MC swing >> from 6.5% to 13% is considerably less than it is from 4% to 13% and >> the level of compression in the rib-crowned board at 13% MC will >> also be less. Hence the rate of compression-set will also be lower. >> And at either extreme there will still be system crown (even if the >> panel itself is in tension). And there will continue to be system >> crown even if cracks should eventually develop. > >> Now in both cases I must point out that the MC swings mentioned are >> the extremes. In most homes they won't go this low or this high >> except, perhaps, under special circumstances and that only >> occasionally. As well, coating the soundboard with a good quality >> lacquer or varnish will also retard the passage of water vapor into >> and out of the soundboard panel. > >> With a rib-crowned soundboard system exposure to any of the typical >> environmental extremes found in the continental US should be >> tolerable. With a compression-crowned soundboard system it is not >> the cracks that are the problem. It is the loss of stiffness due to >> the continual deformation of the wood fibers that is the problem. If >> the piano is going to be kept in an environment that works to >> provide the soundboard with crown and stiffness, damage is going to >> be taking place. The only way to prevent that damage from taking >> place is to keep the piano in an environment that keeps the board in >> an uncompressed condition. Beyond this, the best we can do with a >> solid wood panel is to design the thing to minimize the destructive >> effects of piano's environment while optimizing its acoustical >> potential. > >> Del > > > > -- > > > OVERS PIANOS - SYDNEY Grand Piano Manufacturers > _______________________ > Web http://overspianos.com.au > mailto:ron@overspianos.com.au > _______________________ -- Richard Brekne RPT, N.P.T.F. UiB, Bergen, Norway mailto:rbrekne@broadpark.no http://home.broadpark.no/~rbrekne/ricmain.html http://www.hf.uib.no/grieg/personer/cv_RB.html ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... 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