Compression Question

[Delwin D Fandrich]

----- Original Message ----- 
From: "Bill Ballard" <yardbird@vermontel.net>
To: "Pianotech" <pianotech@ptg.org>
Sent: August 29, 2003 5:14 AM
Subject: Re: Compression Question


> Thanks, John. You've got me started on my own homework assignment.
> When you say, "narrow these figures about 10%", it seems you mean 60%
> RH - 10% RH, instead of 60% * .9. I need to adjust the figure for a
> board ribbed @ 4.5%, on the assumption that was the operating EMC (as
> Del would assume).

I would assume that only if I knew the manufacturer in question was
primarily compression-crowning their soundboards. Then I would assume the
wood MC in the soundboard panel at the time of ribbing would have been
around 4%. Unless it has changed recently Steinway's specification calls
for a MC of 4% summer/3.8% winter.


>
> Regardless of the thread earlier this Spring about the difficulty of
> determining the original manner of board assembly 50-100 years later,
> I'd assume that if it's still showing crown AND the bottom side of
> the rib is flat, the ribs were originally curved. That doesn't rule
> out the possibility that Petroff both crowns their ribs AND assembles
> the board at killer low EMC.

Fewer and fewer manufacturers persist in using a pure compression-crowned
soundboard system. Indeed, Steinway is the only company left that I am
reasonably sure of. A few are using a more-or-less pure rib crown system. I
suspect a study of the subject would reveal that, among those who actually
know enough about soundboard dynamics to understand what they are doing,
most would tell us they are using some form of hybrid system. That is, the
soundboard panels are somewhat drier that I'd like to see and ribs crowned
a bit less that I'd like to see.

A few, and I suspect Petroff is among them (though I've not visited their
factory so have only the visual evidence of their finished soundboards to
go by), seem--as you suggest above--to keep a foot in both camps. They dry
their board way down to compression-crowning levels and they crown their
ribs. At least a little bit. The only good thing to be said for this
practice is that even though the soundboard panel will eventually
self-destruct the system will retain some crown as it does so.

Del