soundboardinstal again

[Isaac sur Noos]

Del,

I suppose we experiment a language barrier limit there .

I was made aware of the use of the word "tension" in English as being
slightly different from there.

The idea is that the board may be constrained while glued in, then a
bit more with the humidity coming back, and that is considered
providing a bit of strings pressure resistance (more fast spring)

Yes we use the term tension to talk about something stretched tight.

About the "flat bars" against the crowned bars method , Stephen agree
that we are expecting more from the wood, across grain in that
process, and it is more prone to flatten with compression set. But
still, why do we have boards, that where made in the beginning of the
last century , with a perfectly good crown and an inflexion point
under the "killer zone" that does not go to the point front bearing
suffers ? (while more recent production are often suffering of a less
than impressive board)

He believe that the way the board is ribbed ,then adapted and glued in
the piano is of much importance, the ribbed panel being kept in it's
"bed" under controlled moisture at any time before the final gluing
(done under controlled moisture too).

For the slant in the belly, he wonder why so many are taking the
effort to compute that slant (Steinway adopted it after some time,
Fazioli made numerous try, Fenner compute them also...)
May be in the end that is overkill, but, particularly because little
differences in placement produce so larges amount of stress in the
system, it should may be not be under considered .

Greetings


Isaac OLEG


> > 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
>
>
>