Inharmonicity and humidity

[Robert Scott]

Newton wrote:

>I originally posted this observation, that FAC values, especially the A,
>change with the humidity changes from 15% to 85% on the same piano in
>the same room.  At a university tuners tune the same piano time after
>time after time and some things become obvious in that setting that do
>not otherwise.

>Water content changes the speed of sound in the wood or the change of
>crown or bearing or change of alien rays.

>When the A changes from 6 to a 9 it is hard to account for such without
>alien rays.

  I believe inharmonicity is almost entirely a function of things
that go on in the string itself.  When the humidity changes as much
as you cited, the swelling or shrinking of the soundboard requires
a significant change in tuning pin rotation to maintain standard
pitch.  This in turn changes the termination point on the string
at the V-bar or agraffe.  If a small segment of string was
previously bent at the V-bar, but now is moved out into the
speaking length of the string because of tuning, that little
bend could certainly have an effect on the inharmonicity - even
if it was only 1/16" long.

-Bob Scott
Ann Arbor, Michigan