Soundboard Torture

[Farrell]

I'm not 100% sure I am understanding your comments correctly, but here goes: The board in my picture has very thin (about 3/8" thick) ribs and offer little resistance to bending. An unribbed panel will expand and contract with changing humidity without crushing - because it can expand freely. With my MC gauge in the photo, the wood is relatively free (compared to a piano soundboard in a strung piano) to expand and contract, so little, if any, crushing is going to occur with my MC gauge (likely no crushing because the level of compression force here is likely well below the failure level of spruce). In the piano, when the soundboard panel is exposed to a similar high humidity environment, the panel will try to expand as much as the panel of my MC gauge. It will tend to crush though because the stiff ribs 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).

Does that address your comment? Are you asking whether panel damage can occur from the rib restraint alone? I don't know what the numbers are, but obviously if the ribs were big and numerous enough, the panel thin enough, and the humidity high enough, I'm sure you could set it up where the panel would fail in areas.

The panel will crush if it is restrained enough. In a strung piano, I don't know what percentage of the restraint is from ribs and how much is from string bearing. I suspect the ribs play a much larger role.

Terry Farrell
  
----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "Pianotech" <pianotech@ptg.org>
Sent: Sunday, March 30, 2003 5:36 AM
Subject: Re: Soundboard Torture


> I thought the argumentation went along the lines that the soundboard
> simply crushes as a result of the compression that resulted from this
> kind of crown pressure alone, and that the added pressure from the
> string bearing simply exsasperates the problem.
> 
> Neat experiment. It shows perhaps more then anything else the importance
> of holding instruments within climatic tolerances.
> 
> RicB
> 
> 
> Farrell wrote:
> 
> >  Soundboard panels in pianos exposed to high humidity are trying to do
> > what you see above. The stiff ribs and string downbearing pressure
> > keep it from actually developing 8 inches of crown. And instead of
> > being able to expand and produce this kind of crown, the soundboard
> > simply crushes. No wonder soundboard panels develop pressure ridges
> > and then cracks when they dry back down! The scale on the left goes
> > from about 5.5% MC to about 8.5% MC. The panel should be in the range
> > of 18% MC in this photo.
> >   Fun stuff! Terry Farrell
> 
> --
> Richard Brekne
> RPT, N.P.T.F.
> UiB, Bergen, Norway
> mailto:rbrekne@broadpark.no
> http://home.broadpark.no/~rbrekne/ricmain.html
> 
> 
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