impedance

[Ron Nossaman]

>
>Yes, point taken, at least as far as production models go. But what a study
>tool it could make! See what _actually happens_ when you change this or that. 
>
>For instance, if you move a soundpost on a violin or a cello a tiny
>fraction of an inch, the whole character of the tone and responsiveness can
>change. What if someone had decided that they could _design_ it to be the
>right place, so they made it so it couldn't be moved? It seems to me that
>"practicality" in piano building has been doing similar things for years.
>That is, the fixed design has meant that any experimentation with piano
>parameters can only be achieved with weeks or months of effort and great
>expense. 
>
>What would happen, for instance, if instead of one long treble bridge all
>screwed and glued in place you had 8 or 10 little ones of a few notes each,
>which were just pressure fit, and held on by string pressure, like a
>violin's? You could move them around and find the RIGHT place for them, or
>see how the character of the tone changed for different places. This might
>enable you to see how much of an instrument's tone was inherent in the
>board, rim, and plate, and how much was an artifact of bridge placement and
>height. It would also be fun if a way could be devised to have movable ribs
>with adjustable stiffness, without having to unstring or lower the string
>tension. By putting pressure on the ribs from underneath instead of
>counting on gluing and clamping to make the board conform, they might be
>made adjustable and movable ... of course one would have to allow for the
>coupling of whatever was pressing on them.
>
>Just gathering wool again ...
>
>Susan


Dear Woolite,

Yea, it sure would be an educational luxury, at this stage, to be able to
shuffle things around like that. It would be a far greater luxury to be able
to mathematically simulate the changes and anticipate the results without
having to pick out the splinters and try again. So build one and find out.

Let's see here... Moving bridges around would change the speaking lengths
and strike point ratios, string lines between agraffe and hitch pins,
tensions, wire sizes (including bass wrap requirements), inharmonicities,
and load distributions. Also, how could you tell how much of the observed
tonal character was a result of the temporary nature of the bridge
attachments point? It seems to introduce as many problems and unknown
variables as it addresses. The same sort of problems occur in the movable
ribs scenario. I'm convinced that at least some, if not most, of the
stiffening of a soundboard assembly under string load is the result of the
panel compressing as the rib is deflected. This happens in crowned rib
assemblies as well as those that are compression crowned, and the panel must
be glued to the rib for it to work. In other words, you could probably learn
a lot about what affects the sound of an acoustic system that, in execution,
is considerably different than a piano, and probably doesn't translate well. 

I think that, once you had configured all this movable stuff to suit you,
and built a prototype in that configuration, you would find that the
interactive physics of the model was so much different than that in the
product that, even though the dimensions correspond, they would sound quite
a bit different. 

We can currently diddle string scaling in software and anticipate, to a
large extent, how it will work in a piano. That's a pretty recent, and way
cool kind of capability, compared to how it was done a hundred years ago.
Why can't we do the same with soundboard assemblies, or actions, or hammer
voicing? If we can determine and model the physics accurately enough, we
could even simulate a physical model of what you described above and not
even have to build the simulator. Then maybe we can finally get some good
out of these electronic autistic savant space heaters taking up so much room
on our deeply piled desk tops. Naaah!

 Ron