Further to Drifting Unisons

[Fred S. Sturm]

Actually there is very little temperature variation in our music 
building. Occasional temporary glitches in individual rooms, but 
overall, remarkable stability. Humidity, on the other hand, can easily 
vary by 20% from one day to the next, and gradually shifts during each 
day. This is due to requirements for HVAC (heating, ventilation and air 
conditioning) that require complete air exchanges in public buildings 
every hour or so (I forget what the interval is, but it may be more than 
once an hour).
	At any rate, my comments about unison drift in response to humidity 
change are not based on any individual piano, but rather on observation 
of many, many pianos over 15 years. I pay most attention to my 
observations in the music department, as I have more knowledge of 
intervening conditions there, and see the instruments more often. I also 
have large numbers of individual models, and am able to observe patterns 
that apply to a Baldwin Hamilton as opposed to a Yamaha U-1, for 
example.
	But I see the same evidence in private homes to a lesser degree, and 
certainly in churches, private schools, and other locales. Sometimes the 
variations are relatively minor (more in the one to two cent range), and 
by no means is there a complete consistency throughout the entire range 
of all instruments, but there is definitely a consistency to finding the 
right string (treble side) drifting farther than the left, if they don't 
sometimes even go in opposite directions. And, as I wrote earlier, this 
can happen to such an extreme degree that the difference is in excess of 
20 cents. The extreme case seems to be mostly in octave seven, but 
sometimes in octaves five and six.
	At some point, I hope to have the time to do more than mere off the 
cuff observation, and chart pitch over an extended period of time on a 
few instruments. I started to do that a while back, but I kept noticing 
other things I wanted to keep an eye on, so I would need to add more 
strings to read (initially, I was just looking at the variation across 
breaks and the like). 
	I'd sure be interested to know if anyone else has made observations 
that come anywhere near tracking mine.
Regards,
Fred S. Sturm, RPT
University of New Mexico

Mark Bolsius wrote:
> 
> Fred,
> You make some interesting observations...
> 
> I wonder if the temperature variations that brought about the humidity
> change may have something to do with it?
> 
> If the pianos that do this are the same ones and other don't, then
> monitoring of temperature as wel as humidity would be necessary, but
> possibly the environments with these "troublesome" pianos may have some
> thermal anomaly happening in them. Remember that temperature change also
> causes humidity to change...
> 
> Just a thought
> 
> Cheers
> Mark Bolsius
> Bolsius Piano Services
> Canberra Australia
> 
> ----------