Piano Rims (rambling post)

[Delwin D Fandrich]

----- Original Message -----
From: "Phillip L Ford" <fordpiano@lycos.com>
To: <pianotech@ptg.org>
Sent: December 28, 2001 2:48 AM
Subject: Piano Rims (rambling post)


> 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.


>
> Should the mass and stiffness be uniform or should it vary and why?

With a so-called 'continuous bent rim' it would be difficult, or at least
inconvenient, to vary mass from one end to the other. Stiffness, of course,
can be varied easily by placing stiffeners appropriately. (These will add a
certain amount to mass as well, but their major effect will be on
stiffness.)

As to why, the idea is to keep as much energy in the soundboard panel as
possible. A light and/or flexible rim will more readily absorb energy from
the soundboard, energy that would be better used in maintaining the sustain
time of the vibrating mechanism.


>
> Some time ago I suggested that bamboo might be an interesting material for
piano
> rims.  Bamboo has very high strength and stiffness relative to its weight.
If you take
> a maple rim as the standard then the bamboo rim could be stiffer and
stronger
> while being lighter.  Would this be a good thing?

Someone will have to try it to find out. If a difference in tone performance
is noted it will probably be in sustain time through the mid-tenor and
treble sections. 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.)

Reducing the mass of the rim assembly would be a good thing in that pianos
are generally quite heavy and anything that can be done to reduce their mass
would be welcome. Or, of course, we can continue to ignore our declining
customer base and watch it decline even further.


>
> My experience is that the pianos I like have massive rims.  However, if
you look at
> wood property charts for American hardwoods you'll notice that density and
> stiffness go hand in hand.  If you pick a denser wood you get a stiffer
wood.  So
> maybe the pertinent thing about the pianos I like is that they have
stiffer rims, not
> more massive rims (the density is perhaps inconsequential).

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. Baldwin switched from maple to
poplar in the rims of some of their pianos a few years back and the change
was noticable, though I don't know if you could trace the change to either
mass or stiffness specifically. At the time the consensus was both but that
was probably based on conventional wisdom. A switch back to maple for the
inner rim was deemed acceptable at the time and they have since gone back to
all maple rims.


>
> Also, the pianos with less massive or
> stiff rims that I don't tend to like tend to be low quality pianos.
Perhaps the reason
> that the sound is less than stellar has to do with the design and quality
of fit and
> finish rather than the fact that the rim is not massive or stiff.  Perhaps
the lightness
> or flexibility of the rim is coincidental.

We've only put new boards in a couple of Yamaha's and the results have not
been what they would have been in similarly-sized pianos using maple (or
similar) rims. The differences have primarily been in sustain time, though
I've not particularly been happy with their overall power either. And that
starts the mad cycle of stiffer, more massive soundboards and heavier,
denser hammers.


>
> If the rim needs to be massive, then why?  I've heard it said that it is
> 'to reflect waves' back into the soundboard.  But I've also seen a few
posts lately
> that suggest that waves don't travel in the soundboard.

If you don't like the idea of 'reflecting waves,' how about just not
absorbing energy. (Or has that been removed from the soundboard as well?)


>
> If the rim needs to be stiff, then why?  Is it to resist the tendency of
the perimeter of
> the board to move outward or inward as the board vibrates?  If the board
is so
> much more compliant than the rim then how does having a stiff rim help
you?  Is it
> to prevent the tendency of the perimeter of the board to move up and down
as the > board vibrates?  If so it would make more sense to make the rim
deeper rather than > thicker, which seems to be contrary to accepted
practice.

What little study and testing of this I've actually done indicates that the
rim tends to rotate around its center of gravity in response to the wave
energy moving the soundboard. And that center of gravity is typically below
the top of the inner rim. I'd not be devastated in finding this is wrong,
but I would be some surprised. With a clamped-edge vibrating panel the
tendency of the vibrating panel is to rotate the clamping mechanism. If the
stiffness and mass of the clamping mechanism are both very high relative to
the vibrating panel its motions will be slight and most of the energy of the
vibrating panel will stay in the panel. If the stiffness and mass of the
clamping mechanism are low relative to the vibrating panel its motion will
be greater and a larger amount of energy will be transferred from the panel
to the clamping mechanism. In either case the motion of the rim is very
small at the point where the soundboard is attached to the rim. Typically,
when we feel a rim and find it moving we place our hand up toward the top
and say, "wow--feel the rim move." Of course, that's the end of the lever.
As far as the soundboard is concerned the top of the inner rim is the
critical area.

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.


>
> If the rim needs to be stiff then why float the board or make undercuts in
the rim
> at certain spots?  Apparently the rim doesn't necessarily have to be stiff
in these
> areas.  Maybe the rim should be stiff or heavy in some areas but not in
others.

Floating and undercutting are two different things.

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.

"Floating" involves actually separating the soundboard from the inner rim
completely. And, yes, by itself it does decrease the stiffness of the
soundboard in the affected area. But, this is typically only done in the
bass region of small pianos where the motion of the soundboard around the
bass bridge would otherwise be overly restricted. Life's a compromise.
Overall system stiffness can be made up for in a variety of ways, if it is
deemed necessary.


>
> 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.

This is one of the primary functions of a good rim bracing scheme. At least
it should be, though often it's not. The typical S&S rim bracing scheme
(including its many clones and semi-clones) does nothing to stabilize the
belly rail in the upper tenor/treble section and is a contributing factor in
the short sustain time of these designs through the upper tenor and lower
treble.

A well-designed rim bracing scheme, of course, will also serve other
functions. It should stiffen the long bass side which can get pretty floppy
even with a fairly massive rim assembly. It should provide mounting points
for the various nosebolts. The function of the cast gray iron bell used in
the larger S&S pianos could probably be better served by a bellybrace
extending from the inside curve of the inner rim to the bellyrail. A
nosebolt could still be used to mass couple the plate to the brace. Nothing
would be lost and much would be gained.


>
> If the rim needs mass then the braces aren't
> helping much.  You'd be better off putting the material into a thicker rim
and
> eliminating the braces.  If the rim needs stiffness then many brace
configurations
> don't help much in certain directions.  Many are good at increasing
stiffness front to
> back but do almost nothing side to side. Do you only want the rim to be
stiff in one
> direction and not the other?

In my opinion, a good bracing scheme will provide stiffness both
front-to-back and side-to-side. Look at the bracing scheme used in the
Walter 190 as an example.


>
> If you want soundboard flexibility is it a good idea to thin the rim as
some of the
> old makers did?  It seems you would transfer energy to the rim which isn't
> seemingly very efficient at transfering it to the air, but perhaps it is -
there's a fair
> amount of > surface area there.

What do you mean by "thin the rim?"


>
> 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'm not sure it is a bad idea in and of itself. The design of the soundboard
itself would also have to change. The soundboard system used in the 'modern'
piano is essentially a clamped-edge, two-dimensional wave-carrying vibrating
panel. What you are describing changes this to an essentially hinged-edge
... vibrating panel. Obviously, the edge stiffness of the panel will change
considerably.

Something resembling this has been done by Marshall & Wendell (sp?) in a
piano designed by Chickering. The inner rim had a relief perhaps 6 to 8 mm
wide between it and the outer rim that extended about half way down the
height of the inner rim--I'd guess about 75 mm. The inner rim was relatively
thin and flexible. I rebuilt one of these pianos some years back and wish I
remembered more about it now. It was a fairly short piano and I recall only
that it ended up with a nice bass for a piano of its size. I don't remember
anything much--either good or bad--about the rest of the scale.

Del