Humidity Change and Unisons

[John Lillico, RPT]

>It seems a few weeks ago there was discussion on left, center and right
>strings responding differently to humidity changes.  I was not able to
>follow all this discussion.
>
>Interesting situation today.  Chickering 5'4" 1937.  On 11-96 raised
>pitch 5 cents (67 degrees, 44% RH); 11-97 raised pitch 2 cents (65 and
>36%); 5-99 raised 8 cents (78 and 30%).
>
>Today tuned at 8 cents sharp (where the piano was at A-4), temp. 84
>degrees F. 72% RH.
>
>Bass was all slightly flat at A-440+8 cents.  Tenor section, right
>strings were 4-6 cents sharp, center was at the offset 8 cents, and the
>left strings were 4-6 cents flat of my offset.
>
>Trebble octaves 5-6 right strings were 8-10 cents sharp, center strings
>about 4-6 cents and left strings were close to the offset.
>
>Octave 7 all strings were sharp with the right string the sharpest by as
>much as 30 cents.
>
>Any explanations?
>
>Larry Messerly, RPT 
>Prescott/Phoenix

Larry,

Done Rose started this discussion. I responded, but there was no follow-up. Guess he thought I was pulling his chain.

My belief is that the plate changes with humidity. Notice how the right string segment is shorter than the others from rear bridge pin to contact with the plate. If the plate expands with humidity, the right string (the shorter segment of the three) takes on more tension. The difference in the tri-chords will be more evident in the tenor than the treble 'cuz of the angle of the plate to the bridge pins, if you get what I mean.

The bass section doesn't play a role in this scenario and is usually uniformly flat.

I was utterly amazed when Dick Bittner broke a treble string during a technical and asked if the adjacent strings would go sharp or flat. What do you think they did?

Next post.

John Lillico, RPT,
Oakville, Ontario