Humidity Change and Unisons

[Paul]

John
What you suggest about the plate being affected infers that it is
hygroscopic like wood. I've worked in machine trades around many kinds of
metal including many types of cast-metal poured into a mold for shaping. I
cannot recall ever hearing  that any metal takes on moisture, or at least
enough moisture to change its shape. Temperature is another matter. But
metal moving temperatures are usually not in a home environment around a
piano. I live in SE Minnesota where humidity changes of 30% and more can
occur daily. Pianos here are always shifting. I figured the pitch difference
between the left, center, right string in a unison is the result of
soundboard and bridge cap movement. When humidity rises, wood takes on more
moisture and becomes bigger. This movement changes the tension of the
strings. Consider the formula for calculating pitch. If you alter any
element of the formula, the resulting pitch will also be altered. A tricord
unison has three different lengths (beyond its speaking length). When the
same amount of tension change is used in pitch calculation for each string,
the shorter string will come out with a higher pitch. I verified this "idea"
in conversation with my math major daughter and physics major son. They
concurred.( although most of the time they were way beyond my math
abilities. They used words I can't even pronounce let alone spell here.)
They helped me dust off my high school algebra and figure this out. This
seems to bear out because the most amount of unison pitch deviation occurs
where the sound board and bridge react the most to humidity fluctuations.

I would like to hear from someone with some metallurgy experience and shed
some light on the affects of moisture on metal shape. I don't know your
backgrounds, so maybe one of you has this experience.
Just my thoughts.

Paul Chick

Don wrote:

> >Hi John,
> >
> >I agree that a broken string will cause the plate to move. But the same
> >effect would happen without a soundboard/wooden support structure. How
does
> >humidity affect the plate? (Other than rust in extreme cases).
> >
> Don,
>
> I was tempted to say, "heavy metal" but........
>
> Throw away the soundboard and bridge and just have your strings and plate.
If the plate doesn't swell with humidity, although I think it might, it will
at least be tilted somewhat through expansion of that upon which it rests.
>
> This may not be as extreme where you have those "adjustable" duplux
doohickeys on some grands though.
>
> John Lillico, RPT
> Oakville, Ontario
>
>
>