Soundboard Torture - and Failure!

[Farrell]

You hit the nail on the head Ron.

To quickly recap: I made a four-foot long by four-inch wide MC gauge with a 3/8" panel and two 3/8-inch tall by 3/4-inch wide ribs. I dried all down to 4 - 5% MC and glued it up flat. When equilibrated in my 45% RH shop, it had about two to three inches of crown to it. I put the gauge in my hot box with a bunch of fairly wet spruce (about 45% MC) and closed the box. RH in box rode steady at about 85% to 90% RH for several days and the ribbed panel developed about eight inches of crown. Then I dropped the RH back down. 

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 Farrell
  
----- Original Message ----- 
From: "Ron Nossaman" <RNossaman@cox.net>
To: "Pianotech" <pianotech@ptg.org>
Sent: Sunday, March 30, 2003 6:48 PM
Subject: Re: Soundboard Torture

> > > and the string downbearing pressure prevent it from expanding (of 
> course, it will expand a little bit, and that is why you might see a 
> several millimeter increase in crown - ! but not 8 inches).
> >
> > And then the addition of string bearing would simply compound, 
> exasperate... further damage the cells with even more compression.

And if you want to see compression set in action, Terry, clamp that 
indicator of your MC gage down to a straight plank with various shims in 
the middle to simulate loading it to about a 4mm crown and put it back in 
the box with the new wood. At least try to clamp it down. It will be tough 
to get it that flat. When you get the rest of the wood below 12%MC and 
start cooking it in the box, take the clamps off the gage and dry it down 
with the rest of the wood. You'll find your gage calibration is useless 
then, because the wood of the panel has been crushed. The calibration may 
very well have changed some after just being submitted to that high 
humidity and bending that far, since the side constrained by the strips 
will suffer some compression set. But it might not be a big deal either. I 
can't say for sure. If you did clamp it to a shallow crown, the longer it 
is under that clamping pressure in that high humidity, the more compression 
set it will suffer, and the lower it will read for a given MC when it's 
un-clamped. This is exactly what happens to a new compression crowned 
soundboard when it's dried to 4%MC, flat ribbed, glued in a piano sitting 
on a factory floor at 70°F and 80%RH with the windows open, and strung. 
Only the clamping pressure on the panel in the piano isn't ever taken off.



>What I dont know is just what combination of humidity and downbearing will 
>push wood cells beyond their compression limits in a traditional 
>compression crowned board. I do know, that S&S claim that as long as the 
>humidity levels are kept within recommended tolereances the soundboard 
>will perform and hold up well.  How they define this exactly is another 
>matter :)
>
>Agreed. And I wonder about the same thing.
>
>Terry Farrell

Try the thing with the gage and see what damage is done immediately and 
permanently. There are no numbers that I know of that will reliably predict 
results of compression crowning techniques. It's a matter of degree. 
Different planks of spruce will shrink, expand, and compress under load to 
differing degrees. Each plank in a every panel will have it's own specific 
and unique stiffness and density, as anyone who has handled un-ribbed 
soundboard panels immediately understands. The whole point of rib crowning 
is that compression crowning doesn't yield controllable and predictable 
results, neither immediately, nor long term. Decent climate control will 
minimize the compression set damage rate, but it won't eliminate it.

Ron N

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