Facts and nots : was Recommend Rebuilder?

[John Ross]

What I was thinking was, that if boards are glued together, then the 
compression takes place. If some of those wood fibers compress, and form the 
ridge, then the measurement, is less than if that portion had not compressed 
more than the rest, so the crown would be less. Or is the crown the same, 
because of the ribs, and the board just came in from the edges?
I hope someone, knows what I am trying to say, as I am having a problem 
expressing myself. Obviously a field, I have not studied.
John M. Ross
Windsor, Nova Scotia, Canada
jrpiano at win.eastlink.ca
----- Original Message ----- 
From: "RicB" <ricb at pianostemmer.no>
To: <pianotech at ptg.org>
Sent: Friday, March 23, 2007 7:11 PM
Subject: Facts and nots : was Recommend Rebuilder?


> Hi John
>
> The whole reason for compression boards is an involved, to say the least, 
> discussion. Compression ridges in themselves do little or nothing to the 
> sound of the instrument... nor do the cracks that eventually show 
> themselves because of these or because of a lack of enough compression 
> when the board was assembled.
>
> What compression ridges show are lengths of the soundboard where the wood 
> has been compressed beyond its tolerance.. the cells are destroyed in the 
> sense that their ability to swell or grow if you will with RH increases 
> to. They are simply crushed.  That in itself is not a big problem when it 
> comes right down to it either... at least acoustically.  A board with 
> severe compression set could be reused as an RC & S panel quite 
> successfully... which makes one ponder a bit on what is actually implied 
> by compression damage.
>
> A board fails to respond acoustically because of other developments in the 
> panel... that can very well be related to compression damage... or because 
> the support against crown may not be sufficient in a given area... or 
> because of  other reasons.
>
> As such... a compression ridge simply shows a weakness in the soundboard 
> that may (and probably will) develop eventually into a crack... and may or 
> may not turn into an acoustic problem given enough time and large enough 
> climatic instability.
>
> At least... thats what I hear from most holds, and it seems to match well 
> with my experience through some 30 odd plus years of working with pianos.
>
> Cheers
> RicB
>
>
>    Speaking of compression boards.
>    What I can't comprehend, is the fact that compression ridges, are
>    apparently
>    ok, by some.
>
>    How can they figure out the crown?
>    Do they know how much of a compression ridge there is going to be,
>    and allow
>    for it?
>
>    This seems to me to be impossible.
>
>    Or is there something, I am missing?
>    John M. Ross
>    Windsor, Nova Scotia, Canada
>    jrpiano at win.eastlink.ca
>