Duplex

[Ron Nossaman]

>Of course. All of the backscale string segments--at least all of them within
>a reasonable distance from the note being played--are affected. 

Exactly, which is why I've wondered for so long about the presumed benefit
of a tuned rear duplex, especially the idea that the often severely
shortened duplex lengths, while limiting soundboard movement, are
increasing sustain and volume.  That must surely be magic. Building systems
with decent soundboards and long free back scales has convinced me, pending
evidence to the contrary, that tuned duplexes are utterly unnecessary. I
can certainly concede that if a system has already been built with a tuned
rear duplex, it probably wouldn't hurt to actually have the duplex in tune,
but I don't see how it is possible for the thing to stay in tune if you did
manage to get it there in the first place. I know that you know this stuff,
but I'd like to lay out a little real world duplex tuning scenario for the
folks at home.

Why does a string segment go out of tune? After the initial stretching and
settling period, it's because of a change in string length. That's it.
Changing string length changes tension, which changes pitch, whether the
length is changed by the tuner, or by heat or humidity induced dimensional
changes of the bridge and soundboard. If we assume it's bridge and
soundboard rise/fall that changes these string lengths, we must assume that
for any given change to the speaking length, the back scale must change
considerably more because the string length change that affected the string
tensions above the bridge is a higher percentage of the overall length of
the segments below the bridge. Because of the friction at the bridge,
limiting string rendering across the bridge these tension changes aren't
equal above and below the bridge. Depending on whether the pitch is going
up, or down, the back scale will be under either higher, or lower tension
than the front scale. Tensions immediately on either side of the bridge
will be the same for any given string, for very brief periods as they pass
in the night. This will happen at different times in the cycle for each
individual string because segment length proportions are different for each
string. If, on the other hand, the friction across the bridge wasn't any
more than that encountered at the counter bearing bar and V bar, the
segments above and below the bridge would have more nearly equal tensions
and the pitches of the rear duplex and speaking lengths would remain more
nearly, but still not precisely proportional. It isn't, and they won't. 



>Why is this
>such a hard concept to grasp?
>-------------------------------------------
>Del

Now THAT is a mystery that may never be solved.

Ron N