Piano Rims (rambling post)

[Delwin D Fandrich]

----- Original Message -----
From: "Phillip L Ford" <fordpiano@lycos.com>
To: <pianotech@ptg.org>
Sent: December 30, 2001 6:00 AM
Subject: Re: Piano Rims (rambling post)


> On Fri, 28 Dec 2001 09:31:26
>  Delwin D Fandrich wrote:
> >
> >----- Original Message -----
> >From: "Phillip L Ford" <fordpiano@lycos.com>
> >
> >> To come somewhat to the point,
> >> I've been pondering whether rims should be massive or stiff or both and
> >why?
> >
> >I doubt anyone has done any exhaustive tests on this. I suspect the
answer
> >lies in the basic mechanical impedance relationship, i.e., mass being
more
> >critical toward the treble, stiffness being more critical toward the
bass.
>
> I've heard you say this before and it's confused me
> every time.  Perhaps I need a little more tutoring
> on this.  I believe you want high impedance in the
> treble and low impedance in the bass.  Is that
> correct?

No. It's just that mass has more effect on high frequency impedance and
stiffness more on low frequency impedance.



> So I would think stiffness would be more critical in
> the treble and mass more critical in the bass.  Or
> in other words, I would think you want to insure that
> you have a lot of stiffness in the treble and you
> have a lot of mass in the bass.  Do I have this
> backwards?

Yes. Try moving a box of tuning pins back and forth slowly. Now try moving
it back and forth quickly.

> >
> >With a so-called 'continuous bent rim' it would be difficult, or at least
> >inconvenient, to vary mass from one end to the other.
>
> Granted.  It does seem like a potentially good idea
> to have the rim thicker in the treble region and
> thinner in the bass region.  Production would be
> an issue.

Of course there is more than one way to build a rim.


>
> >Typically the loss of sustain time due to a low-mass,
> >low-stiffness rim is countered by making the soundboard thicker and the
ribs
> >a bit stiffer. (Though ribs don't really have much effect on tone
> >performance through the last couple of octaves in the treble.)
>
> Why do you say that?  Should the ribs be removed in
> this area?  What then?  Make the board a little
> thicker?

With the bridge so close to the belly rail the stiffness of the soundboard
panel has much more effect on the soundboard system impedance than do the
ribs.



> >
> >I wouldn't necessarily say mass is inconsequential, but would question
how
> >much mass is really required. Again, I doubt whether any tests have been
> >specifically devised to determine this.
>
> I've seen some pianos with very massive rims that
> I've assumed are overkill (especially since the
> particular pianos that I saw weren't great sounding
> pianos).

Well, not all pianos with wonderful rims are well designed pianos. There's
also the string scale, the soundboard scale, the plate, the hammers and
action....


> Since the soundboard is flexible, it seems
> that at some point adding more mass or stiffness
> isn't really helping you since the rim is so much
> more rigid than the soundboard.  This seems to be
> one of those areas like so many others in piano
> design - each maker settled on what he wanted but
> if he did some exhaustive testing to determine some
> ideal (or some point it was pointless to go beyond)
> he never let anyone know about it.

That's quite possible. All of piano design is a compromise, regardless of
which component is being designed. The rim is no exception. Consider the M&H
rim as an example. The rims in these old things are enormously massive and
stiff. I simply can't imagine anything more adding anything to--or should we
say subtracting less from--the performance of the soundboard. Yet, there is
the Centripetal Tension Resonator!

It's possible that someone has done some specific research work on the rim,
but if so no word of it has leaked out as yet.


>
> Also, do you think that damping is a factor?  If
> mass and stiffness alone were what was wanted then
> why not make the rim out of steel.  I've seen
> numerous patents, and a couple of pianos, where
> the soundboard is attached to the plate or a metal
> frame that is attached to the plate.  This idea
> doesn't seem to have caught on.  I wonder if that
> was because of the sound quality, or for some
> other reasons.

Well, the Julius Bauer grand came pretty close to this. There was no inner
rim as such. Just a rim-like structure supporting the soundboard that was
bolted solidly to the cast iron plate. There were so many other unusual
features in the piano it would be difficult to attribute any part of its
performance to its plate/rim configuration.

I suspect cost--and weight--will be the ongoing impediment to steel rims.


> >
> >We are used to building soundboard systems that go along with a
particular
> >type of rim assembly and that is typically stiff and massive. Few of us,
> >unless specializing in replacing soundboards in pianos using Select
Hardwood
> >rims, have much experience with low-mass, compliant rims. The pianos I've
> >seen using low-mass rims still use soundboards of the type used in pianos
> >using high-mass rims, they are just a bit thicker and stiffer.
Personally, I
> >don't care much for the sound of those I've encountered. But that doesn't
> >mean soundboards can't be designed that will work better--at least
'better'
> >by my definition of piano tone--in these pianos.
> >
> >I've been giving some thought of late to various methods of reducing the
> >weight of the modern piano without sacrificing anything tonally. Building
a
> >lighter rim is one way of doing this if the soundboard design can be
> >tailored accordingly.
>
> I take it you're not thinking of making the
> soundboard thicker and the hammers heavier.

No.


>
> >Undercutting will not appreciably affect either the stiffness or the mass
of
> >the rim assembly. Relative to the mass of the inner/outer rim assembly
very
> >little wood is actually removed. The purpose of undercutting is to
enlarge
> >the working soundboard area through the affected area. As to whether this
is
> >advantageous or not is another issue.
>
> I'm not sure that we're talking about the same
> thing.  What I was talking about was the relief
> in the rim that you describe for the Marshall
> Wendell (what I was calling a Chickering).
> This would seem to me to cause a
> reduction in rim stiffness that the soundboard
> sees.  Mass might not be significantly affected.

Ah. I thought by 'undercutting' you were talking about the cove cut taken
around the upper edge of the inner rim to slightly enlarge the working area
of the soundboard. I've no idea what Marshall & Wendell--or
Chickering--called their inner rim arrangement...back-cutting? Feel free to
make something up.

In the piano I worked on this back-cutting (or whatever) extended all the
way around the soundboard. At the time I was just beginning my investigation
into the function of the soundboard and was more interested in what was
happening in the bass than anything else. I suspect any effect this type of
construction would have in the tenor/treble would have been negative. But I
don't remember it well enough to say so with any certainty. If it did, there
is no rule that says this back-cutting would have to extend the full
parameter of the soundboard.

In smaller pianos, and some longer ones if the bridge placement dictates,
we're now floating the bass region of the board. This, of course, takes the
both rim stiffness and mass out of the equation completely.

Del