Bass bridge question

[Delwin D Fandrich]

----- Original Message -----
From: "Calin Tantareanu" <dnu@fx.ro>
To: <pianotech@ptg.org>
Sent: January 23, 2001 7:37 AM
Subject: Bass bridge question


> Can anyone tell me please why in some (overstrung) pianos the bass bridge
is
> quite long, (because the bass strings fan out, I mean, the space between
the bass
> strings is wider at the bridge than at the agraffes), while in other
pianos the bass
> strings run parallel to eachother and the bass bridge is therefore
shorter.
>
> Is there any advantage with one of the methods ot the other?
>
> Curiously, I've seen both types in similar Bechstein painos (both two
meters
> long, one from 1887, with the long bass bridge, one from 1904, with the
> short bass bridge).
>
> Calin Tantareanu
> ----------------------------------------------------

I can't explain the extreme flare you sometimes see -- as in the 1887
Bechstein you mention -- but most bass bridges are flared somewhat to
provide adequate string clearance.

There is a practical limit on how far apart the unison centers can be at the
front termination point. This is limited by how wide the piano is going to
be, how much it is practical to flare the keys, etc. There is also a limit
on how close together the strings of an individual unison can be. By long
tradition the bi-chord string spacing at the agraffe is approximately 6 mm.

>From the agraffe, the first place string spacing becomes critical is right
at the hammer strike point. The unison center spacing is generally a
function of the width of the action components plus whatever space is
required for clearance as they move through their prescribed arcs. The
minimum spacing is generally established by the hammer and is typically
between 14 and 16 mm.

At first glance it would seem like the 6 mm spacing of the agraffe holes
should be adequate, but consider that some bi-chord unisons down toward the
transition between the bi-chords and the mono-chords can be fairly large,
frequently up to 3+ mm. There goes any hope of adequate string clearance.
Coupled with the wild and unpredictable gyrations they go through during
even normal playing conditions, they are going to be colliding with one
another unless something is done to separate them. If the strings are placed
too far apart the hammer either will not strike both strings (of a bi-chord
or tri-chord unison) properly unless we make the hammer wider. But, making
the hammer wider is not a viable option since that would add significantly
to the mass of the hammer. (The bass hammers in an overstrung piano already
have a mass problem since they are typically 7+ mm longer than their
tenor/treble counterparts.)

So a certain amount of flare is built into the string layout.

Since most hammers are approximately 10 to 11 mm wide this means that the
centers of the strings at the hammer strike point can be approximately 7 or
8 mm apart without causing too many problems. (And, let's not forget action
shift...) Now, if the string centers are 6 mm apart at the agraffe and 7 or
8 mm apart at the hammer strike point it is apparent that they are going to
be diverging away from each other as they extend back toward the bass
bridge.

Now, if the unison centerlines were parallel to one another the strings of
the adjacent unisons would be getting closer and closer together as they
approached the bridge. So the unison centers are also flared so that they
also diverge as they approach the bridge.

How much all of this diverging is necessary is a function of the scale
length, scale tensions, etc.

Beyond the minimum required for string clearance, however, I don't know why
some designers have taken the idea to such extremes. I have long suspected
that it was originally associated with the notion that the sound waves
radiated directly from the soundboard and if they could be spread over a
larger area there would be more sound. I'm not aware of anything that has
ever been written about the subject specifically, however. That's just from
reading between a lot of lines.

Del
Delwin D Fandrich
Piano Designer & Builder
Hoquiam, Washington  USA
E.mail:  pianobuilders@olynet.com
Web Site:  http://pianobuilders.olynet.com/