soundboardinstal again

[Delwin D Fandrich]

----- Original Message -----
From: <ranjacob@umich.edu>
To: <oleg-i@noos.fr>; "Pianotech" <pianotech@ptg.org>
Sent: July 29, 2003 11:22 AM
Subject: RE: soundboardinstal again


>
> The reason most commonly given for designing for the prevention of flat
or
> negative crown is, that the coupling of the string to the board assembly
is
> otherwise either likely, or certain, to become
> insecure.

This may have been true at one time. It is now generally understood that
soundboard crown (working against string downforce) is one way of
increasing system stiffness without increasing mass. String coupling is
another issue.


>
> But the prevention of excessive deflection in the board assembly--
> under any humidity and dryness conditions--is also the prevention
> of a certain degree of stiffness of the assembly at the coupling,
> when the string and board are at rest (zero amplitude). This is, in a
> sense, independent of the question of the coupling's security.

I'm not sure I understand this. ...The prevention of excessive
deflection...is also the prevention of a certain degree of stiffness...?
What is preventing excessive deflection and what is preventing stiffness?

Mechanical string coupling to the bridge is a function of the bridge pins,
not the string deflection angle commonly referred to as downbearing.

If you are referring to the relative lack of compression in the soundboard
panel, that is compensated for through the crown and stiffness in the ribs.


>
> This idea of a certain "independence of function" is, I take it,
> what has often led to proposals for a different sort of coupling.

I think there is a fundamental misunderstanding here. (Though I could just
be misreading the above.

The purpose of string downforce working against the crown of the soundboard
system is not to improve mechanical coupling between the strings and the
bridge. The string offset between the bridgepin rows takes care of that.
It's purpose is to set up a system of opposing springs to control the rate
of energy transfer between the strings and the soundboard system.


>
> But it has also led me to wonder about such innovations as
> Grotrian's "counter-bridge", underneath the piano, under the bridge. Or:
> *whatever* Grotrian's own purpose may have been, why not consider having
> one or more counterbridge as a "garantor of crown", in the same sense
that
> Ron's rib set is, and replacing a certain part of that latter function?
> Ribs are still needed for the solid spruce planks, but - perhaps - the
> whole assembly could then be designed without as much of a problem of the
> effects of compression on the panel sub-assembly, i.e., especially the
> vector of compression that runs across (perpendicular to) the plank's
grain?

Again, I may be misunderstanding you , but... The whole idea of building
the crown into the rib is to remove the necessity of creating cross-grain
compression within the soundboard panel. In a properly designed rib-crowned
soundboard assembly the soundboard panel will have been conditioned to a
moisture content that will make it relatively neutral through the normally
anticipated climate changes. Whatever crown exists in the system is there
because it has been machined or otherwise fixed in the ribs. The ribs will
then act as structural, load-carrying members. They are, in essence, long,
curved springs made of wood.


>
> My assumption in all this is, that, in many present designs, the
> planks benefit--as acoustical components--from having a longitudinal,
> end-to-end crown, and that the with the new approach, they could keep
this,
> while otherwise suffering no damage.  Of course, I'm far from sure.

I don't think I understand what you mean by this: "...The planks
benefit--as acoustical components--from having a longitudinal, end-to-end
crown...."

>
> And, what *did* Grotrian have in mind with a "counter-bridge"?

It is difficult, sometimes, to figure out what a piano designer/builder had
in mind when he did designed and built in some specific feature. We can
second-guess all we want, but in the end we can only analyze what is
actually happening in the finished system. Several builders have added
sister bridges on the bottom of the soundboard. They do have the effect of
stiffening the system along the bridge line. They also add mass to the
system. Whether either of these effects is actually desirable is another
issue.

Del