soundboardinstal again

[Delwin D Fandrich]

----- Original Message -----
From: "Isaac sur Noos" <oleg-i@noos.fr>
To: "Pianotech" <pianotech@ptg.org>
Sent: July 20, 2003 2:38 PM
Subject: RE: soundboardinstal again


> Hello,
>
> The soundboard installation as explained to me is forcing a bit more
> from all around, because of the shape of the rim (belly ?) where the
> ribs and the board are glued, the coming back of the moisture in the
> assembly, and possibly the installation from the front with wedges or
> any acceptable method to force the board in place.
>
> On a the few Steinway I could measure I for sure never find a large
> crown in the long direction, but does it mean the approach is useless
> ?

It is not useless, but it is not reliable. The amount of crown you
measure--especially with an unloaded soundboard assembly--will depend on
the weather. In humid weather it will be higher, in dry weather it will be
lower. This will have a considerable effect on how you set string bearing.


>
> I believe that the place where the ribs are glued may be very tight,
> around the panel, I am unsure it should add a lot (was said that the
> little angle given top the belly was very important).

This has been written of at length in the past. I still do not understand
how it is expected that beveling the top of the inner rim and/or the
bellyrail is going to accomplish anything. Just consider the flexibility of
the spruce used for the soundboards and ribs and their relative
thicknesses.


>
> But any method that gives a security against the glue sag (give) is a
> good one.

Glue sag? Are you referring to what we call "creep?" If so, this is only a
factor with a compression-crowned soundboard assembly. The ribs in
compression-crowned soundboard resist the formation of crown--that is, they
try to hold the soundboard flat while the expanding soundboard panel
creates a stress interface between the rib and the panel that actually
forms the crown. Yes, here a lack of glue creep is important. In a
rib-crowned soundboard assembly the crown machined into the rib forms the
crown. Glue creep is relatively unimportant.


>
> The basic resistance and flexion model that where given in the PTJ
> 1968 journal (translation of a writing by Ulrich Laible, one of the
> teachers of the German Ludwisgburg school) are relatively basic and
> based on static material resistance, they don't take in account the
> placement of the neutral fiber in the assembly, the fact that the ribs
> are in compression and the panel in compression and tension, the
> thinner end of the ribs, and so on. A friend of mine have written a
> formula to compute the sag of a soundboard, but after evaluation it
> does not work either, because he see the assembly like a range of "T"
> made in an homogenous material.

What do you mean by, "the neutral fiber in the assembly...?"

It's been many years since I read Mr Laible's article, but wasn't he
treating each rib as a simple structural beam? If so, then the soundboard
panel can be treated as structurally neutral. It does, of course, add mass
to the system. But, since it is (usually) cross-grain to the ribs and both
the compression strength and the tensile strength (across grain) of spruce
are relatively low, it does not add much of any structural strength or
stiffness to the assembly. At least not over the long term as long-term
compression set becomes a factor. (This assumes the ribs were glue on while
the panel was at some "normal" equilibrium moisture content--we glue up
with the panel at 6.5%.)


>
>
> We where just talking about that today, with the conclusion that the
> computation are in fact useful only in the sense that if your ribbing
> is in the parameters computed, the soundboard for sure will be stiff
> enough.
> But so much more data may be taken in account if one want to compute
> the real down bearing counterforce available, that it should be a very
> complicated task.
> The list of parameters that should be added to obtain a figure that
> should be near the reality may be pretty important.
>
> I've been surprised that some rule of thumb does not exist (to my
> knowledge) on how to shape the end of the ribs to get the flexibility
> and the spring effect (and why the ribs where considered as
> rectangular all along in the precedent formula)

I expect that most of the rebuilders designing new rib sets for the
soundboards they install do have these "rules of thumb." It becomes a
combination of science tempered with experience. I have developed a
relatively simple Excel spreadsheet that I use to calculate the specifics
of each rib set. The formulas I use are based simple beam formulas modified
to account for the cross-section shape, the straight tapering, the crown
radii, the projected string loading, etc., that we use. The rib
specifications I get from this spreadsheet are based on years of experience
with the specific types of rib system that I've evolved over the years.
Both the formulas and the process have changed over the years as I've
gained experience with the system. It's far from perfect, but it is close
enough that I can predict with reasonable accuracy the acoustical results
I'm going to get.

My system works only with rib-crowned boards since that is all we are doing
these days.

As I said, I expect most rebuilders doing this kind of work have similar
techniques.


>
> Do some of you tone builders play with the ribs stiffness once they
> are glued on the panel , evaluating what you get by feel and
> craftsmanship ? before gluing the panel ? after ?

Not any more. Early on I did a lot of my experimental soundboard work on a
vertical without backposts and on a grand piano with the belly braces
removed for easy access to the ribs. I could put a soundboard in with
really large ribs and then trim them down to see what would happen.
Otherwise it is pretty tough to get at the ribs.

Del