<!doctype html public "-//W3C//DTD W3 HTML//EN"> <html><head><style type="text/css"></style><title>Re: Soundboard crown</title></head><body> <div>Well put Del. I do hope the doubting Thomas' of the RC school take it on board.</div> <div><br></div> <div>Ron O.</div> <div><br></div> <blockquote type="cite" cite> <blockquote>----- Original Message -----</blockquote> <blockquote><b>From:</b> <a href="mailto:Richard.Brekne@grieg.uib.no">Richard Brekne</a></blockquote> <blockquote><b>To:</b> <a href="mailto:pianotech@ptg.org">Pianotech</a></blockquote> <blockquote><b>Sent:</b> August 18, 2003 1:00 AM</blockquote> <blockquote><b>Subject:</b> Re: Soundboard crown</blockquote> <blockquote><br></blockquote> <blockquote>Hi Del<br> </blockquote> <blockquote>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.<br> </blockquote> <blockquote>RicB<br> </blockquote> </blockquote> <div> </div> <blockquote type="cite" cite>At 12:31 PM -0700 18/8/03, Delwin D Fandrich wrote:</blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">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:</font><br> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">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.</font><br> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">For example, I once looked at an<i><b> Unnamed</b></i> (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 10Ľ 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.</font><br> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">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.</font><br> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">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.)</font></blockquote> <blockquote type="cite" cite><br></blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">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.</font></blockquote> <blockquote type="cite" cite><br></blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">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.</font><br> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">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.</font><br> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">(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.)</font><br> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">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.</font><br> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">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.</font><br> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">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.</font><br> </blockquote> <blockquote type="cite" cite><font face="Comic Sans MS" color="#000080">Del</font></blockquote> <div><br></div> <div><br></div> <x-sigsep><pre>-- </pre></x-sigsep> <div><font size="-2" color="#000000"><br> </font><font face="Times New Roman" size="+1" color="#0000FF"><b>OVERS PIANOS - SYDNEY</b></font></div> <div><font color="#000000"><i>    </i></font><font color="#0000FF"><i> Grand Piano Manufacturers</i></font><font color="#000000"><br>      _______________________</font></div> <div><font size="-2" color="#000000"><br> </font><font color="#000000">Web http://overspianos.com.au<br> mailto:ron@overspianos.com.au<br>      _______________________</font></div> </body> </html>