[CAUT] Unison Drift and Humidity

[James Ellis]

There is an ongoing discussion about left-to-right-string unison drift
associated with changes in relative humidity.  Fred Sturm and I have been
discussing this off list.  I have observed this phenomenon for decades on
end.  What I'm talking about is not due to tuning technique, and it's not
the same in all pianos.  In some pianos, it's excessive.  In other pianos,
it's almost nonexistent.

Generally, I find it to be worse in pianos with solid bridges, and least in
pianos with thin hardwood laminated bridge caps.  In typical cases, I think
it has to do with the expansion and contraction of the bridge due to
humidity changes, and the pattern of the bridge pins relative to the bridge
grometry.  In one case I observe from tuning to tuning, I believe it comes
from shrinking and swelling of the pinblock, but this is an atypical case.
It's not what I usually find, and I think in most cases, this drift does
not come from the pinblock.  In most cases, I think it comes from the
shrinking and swelling of the bridge.

There are so many variations to this phenomenon that I can't say it's any
one certain geometric configuration, but I do believe in most cases it's
due to bridge swelling and shrinking due to changes in relateve humidity as
it relates to the pattern of the bridge pins.  Due to the side bearing, the
slightest change in pin spacing will have a profound effect on the tension
of the string, and if that change in pin spacing is not exactly the same
for all three unison strings, they will go out of tune with each other.

Jim Ellis