Tuning Duplex

[Ron Nossaman]

>Ron,
>I've thought that the change in tension in the portion of the
>string between the aliquot and hitch pin was one of the
>things in the minus column of a tuned duplex versus a 
>system like the Baldwin system where there is no friction
>point between the back of the bridge and the hitch.  In the
>tuned duplex system there's just one more friction point
>and one more string segment whose tension has to be
>equalized, not to mention that you can't set downbearing
>on the fly, so to speak, like you can with the Baldwin
>system.  But I've wondered if friction at the aliquot is
>sufficient to cause the string not to render across it in
>normal tuning and playing so the tension in this small
>string segment near the hitch is irrelevant to tuning
>stability.  What do you think?
>
>Phil F

Hi Phil,
That vertical hitch system is certainly a very nice way to set bearing. It
offers a lower resistance to soundboard movement too because it offers a
comparatively low resistance pivot instead of flexing a string over an
aliquot (X) times per second as the soundboard vibrates. As far as tuning
stability differing significantly from a tuned duplex system, I don't think
there's all that much difference. String segments on either side of an
aliquot, and on either side of a capo for that matter, should (very nearly)
equalize pretty quickly because the string rocks across those friction
points when the note is played, lowering the effective friction at those
points and letting the string render through some. The biggest problem I
see to both the vertical hitch and tuned duplex tuning stability is
something that doesn't have anything to do with either one. It's friction
at the bridge pins which, since there is a lot of contact area with two
sharp bends and hard bearing points, and bridge top contact between to keep
the center segment from flexing so the string could rock on the bearing
points, which are the biggest impediment to equalizing segment tensions.
Actually, there is another point here and that is the total length of the
back scale from rear bridge pin to hitch. For a given change in
soundboard/bridge height relative to the string plane, the shorter the
segment, and the higher the slope angle deviation from a line between the
capo and either the hitch point or aliquot height, the more the tension
will change. Since my current thinking is that since the bridge top carries
the string up and down the bridge pins at the same time and in the same
direction the soundboard rises and falls, the static friction will be
broken between the strings and the bridge pins and the string will render
through the bridge somewhat. This means that a short back scale will reach
a higher tension than a longer back scale as the board rises, and pull more
speaking length through the bridge, making the speaking length go sharper
than it otherwise would. Same in reverse. At any given time, probably no
matter what anyone does or doesn't do, I expect that every single segment
in any given string is at a different tension, changing as we stand
watching it. So unless we take the time and trouble to attempt to
individually manipulate tension levels in every segment of every string
every time we tune, everything has an untuned rear duplex, no matter where
the aliquots are. Even if we do take the time, the tensions change at
different rates with the segment lengths with any climate variation and
everything again has an untuned rear duplex.

Ron N