Buttressed Arch. Question for Ron N.

[Ron Nossaman]

> I think what you're saying is that mass in the rim and on the rim via the
> plate is good. Right?

Right. Rim mass=good.


> Now, the impedance under consideration is the ability to resist energy from
> the board into the rim where it's dissipated, if I'm getting it. So, more
> mass = more impedance = more sustain, huh?

Impedance is a combination of mass and stiffness. In the 
soundboard, the optimal proportion is frequency dependent. In 
the rim, whatever it takes to minimize energy transfer is what 
you want. Stiffness is important, but I've come to suspect 
that mass is the major player once a minimum threshold 
stiffness is met. If rim impedance is high enough that it's 
not unnecessarily absorbing soundboard energy, sustain will be 
longer than in assemblies where the rim is unnecessarily 
absorbing soundboard energy. This one's graded on the curve.


> OK, next. If more mass is good, what about the theoretical possibility of
> having the most massive, densest material as a rim to which the board was
> affixed somehow? Just for theoretical consideration, would that make it
> better theoretically?

I've long thought that reinforced concrete would make an ideal 
rim if you never had to move the piano. Factory tours 
witnessing the pouring-o-the rim probably wouldn't generate 
quite the same level of endorphin secretions in the witnesses 
as does the crackling and popping from a rock maple stack 
being forced into the tight bends of a rim press by of a gang 
of big sweaty guys cranking down giant clamps, but that can't 
be helped. I'll leave the psychological analysis of that image 
to the highly trained, highly paid, and highly insured 
professionals, but I think you get the picture.

Then again, a composite layup or casting, with strategically 
placed bracing and mass loading for the inner rim and belly 
rail, should be quite possible, repeatable, and hard to beat.


> Practically, what better material could be used than what is currently used?
> I mean, it seems like you could actually get about anything to work, but the
> heavier you make 'em, the less likely they would be bought b/c they would be
> so difficult to move.

Life's little trade-offs. In practice, you don't have to 
produce an ideal anything. You just have to generate the 
impression that yours is shinier than theirs to sell it, be it 
through "features", or marketing - rarely through actual 
performance. If you want to do something real and marketing 
independent though, good old solid and heavy rock maple makes 
a really nice rim, especially in the belly rail. Birch, beech, 
or something similarly stiff and heavy should work as well. 
Add plenty of maple, oak, ash, birch, beach, or otherwise 
heavy beam bracing all around, and the acoustic foundation, or 
stage, is in place. Pattern routing and stacking dense plywood 
sheets is another possibility for making a nice dense 
integrated inner rim and bracing. This makes it possible to 
define a workable treble soundboard perimeter with rim shape 
(or added fish), and limit rib length with a bass cutoff, 
which won't be wasted by an energy sponge rim sucking the life 
out of the system. Then you can get on with the polish that 
includes string scale, bridge placement, rib scale and such, 
that puts the players on the stage. I start with the string 
scale when it's in the design stage, but I start at the bottom 
with the rim and bracing when it's time to build.

There are a whole lot of possibilities that haven't been tried 
yet, including what may prove to be the better way that is 
just waiting to be discovered.


> Thanks from someone whose head impedance is pretty high. <g>
> 
> John Formsma

Likewise. Mine doesn't ring at all (other than internally) 
when whacked.
Ron N