---------------------- multipart/alternative attachment Well put Del. I do hope the doubting Thomas' of the RC school take it on boa= rd. Ron O. >----- Original Message ----- >From: <mailto:Richard.Brekne@grieg.uib.no>Richard Brekne >To: <mailto:pianotech@ptg.org>Pianotech >Sent: August 18, 2003 1:00 AM >Subject: Re: Soundboard crown > >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 10=BC >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 _______________________ ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/c6/cc/f8/6c/attachment.htm ---------------------- multipart/alternative attachment--
This PTG archive page provided courtesy of Moy Piano Service, LLC