Negative bearing

[Greg Newell]

Del,
    My thanks to you and Ron for somewhat demystifying this for me. No
substitute for getting your hand dirty and doing a few but I'll get there
eventually. I'm still working on obtaining usable shop space. For now though
it's good to gain some understanding of what I'm hearing in the field.
    What was the effect of the floating soundboard across the bottom? I've often
wondered why it's necessary to lock it down everywhere. 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.
    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? OK, now about the long bridge. Should the long bridge
accurately and strictly follow the rim shape throughout it's length? 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?
    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. 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?

Greg

Greg

Delwin D Fandrich wrote:

> ----- Original Message -----
> From: "Greg Newell" <gnewell@ameritech.net>
> To: <pianotech@ptg.org>
> Sent: December 01, 2001 8:16 PM
> Subject: Re: Negative bearing
>
> > Ron,
> >     What are you doing with a new board to eliminate the need for a
> cantilevered
> > bass bridge? Are you thinning the board or routing it in proximity to the
> > location of the bass bridge to make it more flexible in that area? Is not
> the
> > reason for the cantilevered design to get the bulk of the bridge away from
> the
> > edge of the board? Just trying to gain understanding.
> >
> > Greg
> > -----------------------
>
> I'm not Ron, but...
>
> The cantilever was introduced to allow the mounting point of the bridge to
> be placed away from the inner rim and out toward a "more resonant portion of
> the soundboard" while allowing the longest possible speaking length in a
> shorter piano. (And we'll leave the idea of soundboard resonance alone for
> now.)
>
> Since designing a piano of any size with the longest possible speaking
> length is going to place the bridge well back toward the plate hitchpin
> riser, the string backscale is going to be very short. It is not just the
> inefficiency and filtering effect of the bridge cantilever that is the
> problem, an even bigger problem is the shortness of the backscale tying the
> bridge down to the plate. To produce any kind of fundamental energy in the
> waveform, the bridge must have some degree of mobility. That means it must
> be able to move, and with it the soundboard. If the string backscale is
> tying the bridge to the plate it does not have this mobility.
>
> Converting to vertical hitch pins through the bass will partially alleviate
> this problem, but an even better solution is to shorten the speaking length
> somewhat and move the bridge forward some. A typical change in say, a 179 cm
> (approx. 5' 10 1/2") grand will yield a backscale length of from just under
> 50 mm up to approximately 100 mm.
>
> If it is still considered desirable to free the bridge from the soundboard
> in some way it can always be undercut somewhat. This serves the same
> function without the filtering effect, the distortion and the mass added by
> the cantilever. I used this expediency in both the Baldwin 148 and the
> Walter 190.
>
> Yes, the back of the soundboard can also be cut away--'floated'--to free up
> the board more than is possible by just thinning out the soundboard panel.
> We are now doing this on almost all pianos under 190 cm (6' 3") and,
> occasionally, on somewhat longer pianos. It depends on the configuration of
> the bass bridge relative to the inner rim. It's a bit scary the first time
> you do it--the problem is possibly making the soundboard assembly too free,
> or flexible--but after a while you get a feel for what you're doing and the
> results are quite predictable. (A note, our upright piano had a
> fully-floating soundboard across the bottom of the piano.)
>
> Del

--
Greg Newell
mailto:gnewell@ameritech.net