Negative bearing

[Delwin D Fandrich]

----- Original Message -----
From: "Greg Newell" <gnewell@ameritech.net>
To: <pianotech@ptg.org>
Sent: December 01, 2001 9:46 PM
Subject: Re: Negative bearing


>     What was the effect of the floating soundboard across the bottom? I've
often
> wondered why it's necessary to lock it down everywhere.

It opens up the sound of the bass a lot. That is, the sound has much more
energy in the fundamental and the lower harmonics.

It's not, and this is not a new idea nor is it original with me. We've gone
into this before on pianotech and you might want to check the archives, but
briefly it has been done, at least partially, on a number of old grands and
verticals. More recently it was done on Rippen verticals.



> Has anyone ever
> explored a different material joining the soundboard to the rim? I'm
thinking of cone
> type stereo speakers where there is a foam rubber of sorts joining the
cone
> shaped paper element to the stiffer steel frame. It seems to me that there
are
> some parallels here. Probably the tapering of a soundboard is the closest
I've
> heard to this.

Yes. There are a number of patents that have been issued on the subject.
Several of them have the soundboard looking like it was attached to the rim
with a system quite similar to the loudpeaker surround. Another idea that
was used years ago in the Marshall & Wendell (designed by Chickering) was to
use a cut between the inner and outer rims that extended down from the top
of the inner rim about half the depth of the inner rim. Kind of a partially
hinged mounting system.

The typical 'modern' piano soundboard is mounted using what amounts to a
clamped edge system. The various types we are discussing seeks to alter
that, at least in some areas, making it hinged, partially hinged or
free-floating. There are advantages and disadvantages to each.



>     Since I've broached the subject of tapering I'd like to ask if there
are
> advantages to tapering in some areas and not in others. I understand that
it is
> desirable to have a stiffer board in the treble than in the bass. Has any
> attempt been made to make the taper gradual from, say, one end of the long
> bridge to the other?

Yes. In fact, many, if not most, upright piano soundboards are tapered this
way. It's fairly easy to do if you can afford a $75,000+ thickness sander.
It has been done on grand soundboards as well, but I think the more usual
practice is to taper just around the lower half or third of the board in the
bass region. Tapering doesn't have much effect in those areas where a direct
line from the bridge to the rim goes across grain. The 'solid' spruce
soundboard is fairly flexible in that direction anyway. It is much more
effective where that direct line goes with the grain. I.e., from the lower
region of the bass bridge back to the inner rim. Remember that the stiffness
of the soundboard panel is going to increase with the square of its
thickness. Any thinning done there is going to have quite an effect on the
ability of the bridge/soundboard to move.



> OK, now about the long bridge. Should the long bridge
> accurately and strictly follow the rim shape throughout it's length?

I don't see why. In practice they don't.



> Could one
> move the long bridge gradually in more toward the center of the board
toward the
> tenor end  as a replacement for (or compliment to) a gradual thinning of
the
> board toward the same end of the long bridge?

Generally the center of bridge load does just that. Keeping in mind that
this center will get pretty hard to pinpoint as the two bridges of the
overstrung scale interact. One of the sweetest pianos I've ever encountered
was a Chickering flatstrung piano which did have the bridges converging on
the center of the soundboard (more-or-less). Difficult bridge to drill and
notch, though.



>     If the previous is done would it then become impossible to have the
low end
> of the treble bridge and the bass bridge be roughly the same distance from
the
> edge of the board? I believe I understand that if they are not relatively
the
> same distance from the edge of the board that an impedance imbalance
problem
> will result.

In theory you'd like to have the low end of the tenor bridge and the upper
end of the bass bridge more-or-less eqidistent from the inner rim. There are
a lot of variables here, though.

> I have an A.B. Chase I am delivering soon that has a low tenor
> "hockey stick" with all wound strings on it. BOOM, BOOM, BOOM go all the
notes
> down there. Really quite unfortunate but it was here for refinishing. That
> portion of the bridge is curled inward toward the center of the board
(where
> else would it go?) and is quite a bit farther in than the bass bridge. The
> transition is astounding! And not in a good way. Any thoughts? Or am I
just
> stating what you guys have seen for years?

Yes, it's a fairly common problem. I wouldn't say it's the fault of the
wrapped strings on the end of the bridge, though, unless they are badly
scaled. Even if the scaling is perfect there is still going to be a problem
as you describe the system. It sounds like the lower end of the tenor bridge
is overly free to move. That is, the bridge/soundboard/rib system is overly
flexible right there. If you were rebuilding the piano there is a lot you
could do, but otherwise your choices are more limited. All is still not
lost, however. Probably the easiest thing to try is to try adding some mass
to the back of the soundboard at the end of the bridge. This obviously adds
mass when it's probably best to add stiffness, but it's a lot easier to do.
In one severe case I fitted a short rib between two existing ribs to add
stiffness to one of these pianos. This rib was only about 400 or 500 mm long
and was centered right at the point where the last set of strings crossed
the bridge. That worked quite well too.

Good luck.

Del