Piano Rims (rambling post)

[Ron Nossaman]

>To come somewhat to the point,
>I've been pondering whether rims should be massive or stiff or both and why?
>Should the mass and stiffness be uniform or should it vary and why?

My call is both massive and stiff. Lots of bracing. High impedance, as
positive a soundboard perimeter termination as possible. Taking the rim out
of the acoustic mix makes the system easier to anticipate from a design
standpoint. A light and flexible rim robs power from the soundboard. If
that's what the designer wants, great. I don't.  


>If the rim needs to be stiff then why float the board or make undercuts in 
>the rim
>at certain spots?  

Control. Soundboard impedance requirements aren't nearly the same in the
high treble as they are in the low bass. The stiff rim gives you a baseline
from which to calibrate the rib scale, panel, and bridge placement. If rim
stiffness is an unknown, it becomes more of a design crap shoot because you
have an arbitrary foundation. A high impedance rim takes one important
variable out of the loop. Given a dependably high impedance rim, you can
tailor and more accurately anticipate the soundboard assembly design to
produce the approximate response you want at all points in the scale. A
flexible rim won't produce the same effect as floating the bass, and will
kill sustain in the high end unless the soundboard assembly is made
significantly stiffer, which requires harder hammers to produce a given
sound level, etc.


>How do the rim braces fit into this? I've heard it said that they are only 
>there to
>keep the belly rail from rolling.  

Stiffness. A stiff and excessively braced rim and belly rail is less a
vibrational sink than just a stiff rim and belly rail. By my design
philosophy, a rim can't be too stiff. Particularly the long side and belly
rail, but generally overall. This was something Del stressed when I was
experimenting with this stuff, and I found it to be right on the money. 


>If you want soundboard flexibility is it a good idea to thin the rim as some 
>of the
>old makers did? 

Soundboards can be made as flexible or stiff as you need without this. This
rim thinning would depend on rim shape and bridge placement.


 >Perhaps you could carry this to an extreme and have the soundboard resting on
>a thin knife edge around the perimeter, as in a guitar or violin.  Why would 
>this
>be a bad idea?

I don't know, why? As long as the rim is sufficiently stiff and massive and
the gluing surface was adequate to anchor the board, why not? Functionally,
there would be no appreciable difference if there were 5 millimeters or 500
millimeters of soundboard securely glued to the rim. 


Ron N