Plywood shrinkage??? Laminated soundboards???

[Overs Pianos]

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Hi Sarah,

I would like to add a couple of comments re your observations, 
additional to Del's recent post.

>My friend and I were examining some antique furniture, figuring out 
>how we are going to restore it.  I noticed that many drawer bottoms 
>in several pieces were made of thin plywood and, further, that the 
>plywood panels were almost universally shrunken by perhaps 1% of 
>their original size (e.g. 1/4" over a 25" span).

The draw bottoms were most likely not coated with any finish to slow 
moisture transmission, and secondly they would be manufactured from 
material which was sliced through the flitch, so the grain 
orientation would be almost certainly 'all over the shop'. I don't 
believe that one should 'draw' any conclusions on the expected 
performance of a laminated sound board panel based on the shrinkage 
you observed.

>   Moreover, the greater shrinkage seemed to be in width, rather than 
>in length, with regard to the direction of the grain of the outer 
>veneer.

Yes you would expect this, since the draw base was probably of three 
ply construction with the outer laminates running parallel to the 
sides, with just one centre lamination running across. The greater 
shrinkeage therefore would be against the center lamination.

In a solid wood piece the shrinkage would be more pronounced across 
the grain. In the laminated 3 ply piece the 2 outer plies will be 
trying to shrink against the 1 inner ply, while the 1 inner ply will 
be trying to shrink against the long grain of the 2 outer 
laminations. Clearly, the two outer laminations will have greater 
success at contracting the long grain of the inner lamination.

Returning to laminated sound boards, I believe that to reduce 
shrinkage the laminations should be made from quartered material, and 
the panel should be sealed all over. I have seen pianos where the 
upper surface of the sound board was coated purely as a cosmetic 
measure, since the suspended bass bridge had unsealed sound board 
exposed on the underside of the bride apron. The uncoated area would 
act as an effective moisture conduit to ensure poor tuning stability 
in the bass. For the same reason the bridge notching and bridge tops 
should also be sealed. We mask off the inner rim to sound board 
gluing area (as is common) before spraying the underside of the 
board, and when installing the board we apply glue to the rib 
mortices and all around the rib ends (completely covering all 
previously masked areas). Although our ribs are clear of the mortices 
at the sides as well as the bottom of the cutout, we apply glue to 
ensure that the entire sound board area is sealed. The set bolt holes 
are sealed  also.

As Del mentioned, all laminated panels should be rib crowned. A few 
on this list have relayed comments from manufacturers who have 
claimed that RC boards are tonally inferior to CC boards. Often 
laminated panels have been lumped in with similar criticism. Based on 
our own experience at this time I do not accept these claims. I would 
be very surprised if those claimants have actually built a RC 
laminated board as a comparison (I haven't actually built an example 
of each either, but I've worked on compression crowned boards for 
almost my entire 29 years working on pianos). I suspect that their 
comments are based more on a perceived need to criticise any method 
which is different to the one they are currently using and have 
always used.

Ron O.
-- 
OVERS PIANOS - SYDNEY
    Grand Piano Manufacturers
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Web http://overspianos.com.au
mailto:info@overspianos.com.au
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