Humidity Change and Unisons

[Ron Nossaman]

>The sharpness of the tri-chord's right string at this time of year has to be 
>on account of the shorter distance from bridge to plate. If there's a tail 
>end duplex and the right string is sharper than the left, I think I'll go 
>into politics.
>
>John Lillico, RPT


Everything in a piano moves all the time, so anything you could name will
be contributory in some direction in some infinitesimal manner. The bulk of
this stuff happens because the string friction at the bridge is so much
higher than at the aliquots, V bar, and counterbearing bar. The bridge is
the dividing point. With any bridge height change relative to the plate,
the total string length behind the bridge (rear bridge pin to hitch) is
shorter, and changes in tension more than the total string length (front
bridge pin to tuning pin) in front of the bridge. In a unison, the shortest
of the three lengths behind the bridge changes most among those lengths
behind the bridge, and the same happens among the lengths in front. When
the tension differences in a single string are far enough apart on either
side of the bridge, the friction across the bridge will be overcome
somewhat and enough string will render through to get those tension
differences close enough that the bridge friction will again hold them. The
shorter the back scale, the more the front tension will change with a given
bridge height displacement. Also, with identical back scale lengths within
a unison, the string with the shorter total front length will change
tension in that front length more than the string with the longer front
length. It's quite possible, depending on where in the humidity cycle the
piano was last tuned, how nearly equalized the tensions were between front
and back scales when the tuning was done (which you can't know), where in
the cycle it is now, and where in the cycle(s) it has BEEN since it was
last tuned, for the unison to be in tune, right string flat of center flat
of left, right string sharp of center sharp of left, or anywhere from here
to there.

There is absolutely no way to make any sort of reasonable assessment of
what's happening in any specific instance with nothing more to go on than
pitch, temperature and humidity records. Just as well vote for the guy with
the coolest looking suit - as long as you're going into politics. Another
little tip from your Uncle Wookie.

Or maybe it IS the casters or fallboard knobs. I'll leave it to Jim B to
soak us up some knobs and report back later.

Ron N