[CAUT] Unison Drift and Humidity

[Joe And Penny Goss]

Thanks Jim,
 This is a golden nugget to think on.
Joe Goss RPT
Mother Goose Tools
imatunr at srvinet.com
www.mothergoosetools.com
----- Original Message ----- 
From: "James Ellis" <claviers at nxs.net>
To: <caut at ptg.org>
Sent: Wednesday, March 08, 2006 9:00 AM
Subject: [CAUT] Unison Drift and Humidity


> 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
>
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