[CAUT] Unison Drift and Humidity

[Don Mannino]

Jim,

Thanks for contributing to this.  Very thought provoking post.

Ron Nossaman has talked about bridge cap shrinking / swelling as well,
with the resulting apparent shortening and lengthening of the exposed
portion of the bridge pins.  

The grain direction of the wood in the bridge cap would also bear on
both of these issues, with vertical grain (quarter-sawn) caps moving the
most vertically, and horizontal grain moving more horizontally.  It
seems to me that these would affect how the strings would drift in
tuning as well.

Don Mannino RPT

> -----Original Message-----
> From: caut-bounces at ptg.org [mailto:caut-bounces at ptg.org] On 
> Behalf Of James Ellis
> Sent: Wednesday, March 08, 2006 8:00 AM
> To: caut at ptg.org
> 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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