Bridge Pressure Bar

[Delwin D. Fandrich]

----- Original Message -----
From: "Alan Forsyth" <alanforsyth@fortune4.fsnet.co.uk>
To: "Pianotech" <pianotech@ptg.org>
Sent: Monday, April 14, 2003 5:42 PM
Subject: Bridge Pressure Bar


> Del, Thanks for your explanation of soundboard impedance.
> In your other posting you wrote as follows;
>
>   [ "The soundboard system will be more flexible if it does not have a
> string plane bearing against it. It will be stiffer if it does."  and "The
> whole idea of loading the soundboard with string bearing is to make it
> stiffer (or "springier")." and then in a later message "Assuming, of
course,
> that its string scaling, its soundboard and rib system and its supporting
> structure (soundboard liners, back assembly, etc.) is
> otherwise relatively conventional." ]
>
> If stiffness was designed into the soundboard in the first place would
there
> be a need for string pressure?

I don't know for sure but I think so. The soundboard/string system works on
the principle of apposing springs. This being the case there is a range
through which it works most efficiently. As to whether this function could
be duplicated with equal efficiency with a completely flat board and zero
string bearing is something I've not yet investigated.


>
> Here is a picture of the cross section of the soundboard that goes with
this
> pressure bar (image also attached) system. As you can see it IS
> unconventional. The soundboard is double skinned, the outer panels being
> only 2mm thick. The overall thickness of the board is 15mm. The ribs are
> 23mm X 11mm placed 33mm apart. Because of the double skin, this board is
> very stiff, and very light. It is also my intuitive guess that this
> soundboard would be fairly immune to humidity changes - the theory being
> that, I suppose, the less material there is, the less moisture it can
absorb
> or release and therefore the less it will tend to "heave".

How interesting. A modification of the torsion box. Are the two outside
skins a single ply of wood or are they laminated? As shown it appears that
it would be considerably stiffer around the parameter than I would like, but
otherwise does have some potential. Why is it cut away? Is this the way it
was actually installed? Or is this for demonstration purposes?


>
> Sorry I don't have higher resolution pictures, but I am only a Tuner and
> can't afford such fancy gizmos to provide such.
>
> You see, I am just trying to figure out why after 300 years of piano
> construction, the problem of the killer octave and soundboard failure has
> not yet been solved. Make the soundboard with the desired impedance built
> into it before anything else, then use a system of string to bridge
contact
> that doesn't rely on downbearing and hey presto.......no additional stress
> on the soundboard, no killer octave......but I suppose it is more
> complicated than that!

Well, it has been solved or, if not "solved" at least brought under
considerable control. The solution involves cleaning up the stringing scale,
redesigning the soundboard system and the soundboard mounting system, but it
is not all that difficult.

Del