soundboardinstal again

[Delwin D Fandrich]

----- Original Message -----
From: "Isaac sur Noos" <oleg-i@noos.fr>
To: "Pianotech" <pianotech@ptg.org>
Sent: July 21, 2003 2:30 AM
Subject: RE: soundboardinstal again


>
> The inserting under tension of the soundboard seem the only way to
> have some tension in the panel, in the rib direction (across grain)
> the modulus of elasticity is said to be 3 Kg cm2 vs. 100 Kg cm2 in
> direction of grain.
> So if we wish to have some tension in the panel that looks like a
> convenient method, more than compression crowning.

Do you mean tension as in "a pulling force, or stretching something tight?"
Or compression as in, "pressing together by an applied force?"

Generally soundboard panels are under tension only after some period of
having been subjected to compression-set and then finding themselves in a
very dry environment.


>
> Stephen have forced back some boards with shims on the straight side
> with good results he say, when we see the dimensions involved, that
> looks like a possibility.

The problem with this practice is that it works--but only very temporarily.
The physical structure of the wood we use for piano soundboards and ribs is
simply incapable of resisting the stresses created with these techniques.
Entire theories have been built based on the assumption that wood is a
perfectly rigid and non-compressible material. But it is not. Even along
the grain it is a fairly readily compressible material.

The mechanical characteristic involved here is actually compression
parallel to grain, not the MOE. This value, for spruce (it varies with each
specific species) is approximately 5,600 psi to 6,000 psi (36 000 - 40 000
kPa). This is not very much considering the small
cross-section--approximately 0.25 in2 (160 mm2)--of the typical
feathered-out rib end.


>
> The panel is said to be shaped with 1 cm more than the internal size
> of the rim, was said. If possible a soundboard that should be pinched
> in the rim should work better from an acoustical point of view (a very
> good joke to future rebuilders !)

A good joke on the builder as well, I think. Make a sketch of a grand piano
rim, sketch in the ribs and the soundboard grain angle and then think about
the stresses involved with the proceedure you describe. Compare those
stresses with the known physical qualities of wood and you'll see what I
mean.


>
> Idem on the belly rail, while some brands have stopped the insertion
> there (Seiler for instance).
> But I seem to understand that in the actual conception in the USA the
> soundboard assembly is viewed as an auto supported device is not it ?

In the end they are all auto-supported devices regardless of how they were
installed. A few years of compression-set insure this.

Del