Humidity Change and Unisons

[Kevin E. Ramsey RPT]

>From bridge to plate, then how can you account for one string of a tri-chord
being 100 cents sharper than the other side, (which goes to the other note?)
-----Original Message-----
From: John Lillico, RPT <staytuned@idirect.com>
To: pianotech@ptg.org <pianotech@ptg.org>
Date: Sunday, August 20, 2000 6:03 PM
Subject: Re: Humidity Change and Unisons


>Such a long post I usually never read for I thought it was billbrpt! So,
what's the answer to Don's observation about the right string being sharp on
both grands and upright tri-chords? I still say it's the angle of the plate.
On the other hand, vote for me! I'll find the answer!!!
>
>John L.
>Oakville
>
>>>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
>
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