Soundboard crown

[Overs Pianos]

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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


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