----- Original Message ----- From: "Farrell" <mfarrel2@tampabay.rr.com> To: "Pianotech" <pianotech@ptg.org> Sent: Wednesday, April 09, 2003 7:00 PM Subject: Re: Soundboard Torture - and Failure! > > My gauge has been in the shop now for a week at 45% RH. It did not settle back at the two/three-inch crown position. It is nearly straight, with only about ONE INCH of crown. I thought that because the ribs are so thin it would bend quite a bit (which it did) at high humidity, but not crush the spruce panel. Apparently, the panel suffered quite a bit a damage (compression set I suppose) with just the one several day exposure to high humidity. Just imagine what that would do to a piano soundboard that has one-inch by one-inch ribs and all the strings only allowing it to expand a tiny bit. CRUSH, CRUSH, CRUSH! > > I'm getting the to the point where I think a good piano should spent its life with museum-like environmental control - nothing less will do. > Terry, I was debating with myself as to whether or not I should warn you about this, ultimately deciding that experience is a really great teacher and you weren't jeopardizing any priceless pianos on your road to discovery. You can use a spruce panel as a humidity indicator but you have to allow it to free-float. I.e., you cannot allow it to come under compression. Yes, what you are witnessing is 'compression set.' This phenomenon takes place any time wood cells are placed under compression and are particularly observable when the compression is applied perpendicular to grain. It is difficult for people who have a nearly religious belief in viability of the compression-crowned soundboard to understand just how devastating compression set can be to the stability of the completed soundboard panel. Or how quickly compression set can occur. Your little experiment graphically illustrates both the speed and extent to which compression set can occur. Actually, compression set follows a log curve. If you place a sample under a given amount of compression, compression set starts to deform the wood fibers immediately. This, of course, reduces the amount of compression (pressure on the fibers) and the rate of compression set decreases. Ultimately, the pressure against the wood cells will deform them to such an extent that further compression set is reduced to virtually (though not exactly) nothing. Keeping a piano with a compression-crowned soundboard in a hermetically sealed environment would not relieve the problem unless the atmosphere in the hermetically sealed environment were maintained at such a level as to keep the soundboard panel at 4% MC. In this environment, of course, the soundboard assembly would not have any crown. The problem of compression set is present any time a panel such as a spruce soundboard panel is placed under perpendicular-to-grain compression and this occurs immediately upon returning the newly-ribbed soundboard assembly to normal atmosphere. It is only exacerbated by installing a bridge, gluing the assembly into a piano and loading it with some amount of string bearing. Del
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