Humidity Change and Unisons

[Greg Newell]

Paul,
    I have to agree with you here. I found a web page of a guy who "educates"
people by telling them that the sole reason a piano goes out of tune is from the
plate moving. Now I'm no genius but I just can't see how a cast iron frame can
take on enough humidity to change size or shape without visible signs of a great
deal of rust. Temperature is another thing but as you pointed out the extremes
necessary to have a noticeable pitch change aren't normally found in places
where pianos are kept. I really don't understand this position of the plate
having much to do with pitch changes. Perhaps someone could quantify all this
for us by setting up something to accurately measure plate variances in
thousandths of an inch or mm. I wonder, if plate movement is caused by moisture
or heat then why haven't we heard something about this from the good people at
Damp-Chaser? It seems that with all the measuring that they have done they would
have found this phenomenon by now, no?
My 2 cents.

Greg Newell

Paul wrote:

> 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
> >
> >
> >