Humidity Change and Unisons

[Kevin E. Ramsey RPT]

Hi there, I just HAVE to throw in my two cents worth, having read this
thread a little late in the game. I've thought about this, as I'm sure we
all have while tuning, and here's what I believe is happening; I think any
movement of the plate is minimal here, what's really moving is the
soundboard. As the soundboard crown increases-decreases due to changes in
R.H., the tension on the strings changes. It won't effect the tuning until
it reaches a certain pitch, usually about ten cents. When the strings get to
that tension level they end up rendering through the upper bearing point
 Aggraffes or v-bar). Beyond that you have the tuning pins with three
different lengths of string per note. Those three different waste lengths
somehow pull the wire through at different rates due to the shortness of the
strings, the shorter having less "give" perhaps than the longer ones.
    I really hope you don't think I'm all wet here, but this just seems to
make the most sense to me. I think I heard Jim Coleman explain this in one
of his classes, I'm not sure. But it's the soundboard that moves, not the
plate.
    Oh, and about the question of a string breaking, and what happens to the
pitch of the adjacent strings; They go sharp. The bridge has the same
tension on it, but with fewer strings the remaining strings have to absorb
that tension. When they do that, they go sharp. I've seen this, and measured
it, so on this one I know that it happens.
-----Original Message-----
From: Paul <tunenbww@clear.lakes.com>
To: pianotech@ptg.org <pianotech@ptg.org>
Date: Friday, August 18, 2000 6:17 AM
Subject: Re: Humidity Change and Unisons


>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
>>
>>
>>
>
>