[CAUT] temperature and pitch

[Fred Sturm]

On Dec 12, 2009, at 8:53 AM, Jeff Tanner wrote:

> Did you measure RH change as well? Your conditions would probably be  
> different, since you guys don't have any moisture in the air out  
> there. See, here, you can't answer the temperature question because  
> humidity change is so unpredictable. In our situation, you could  
> have a significant change in both temp and humidity during the day,  
> and a piano tuned first thing in the morning would be out of tune  
> enough that a relatively significant pitch adjustment would be  
> necessary in the afternoon.


	Which is precisely why I posted this. It seemed to be an ideal  
example of only the one variable, and over enough time that both  
strings and plate had time to stabilize at the new condition. So  
please don't interpret it as meaning something different. It does not  
say anything about the changes we get when the temperature is in the  
process of changing, where most of us agree there is a rapid response  
by the strings, and especially the bass strings. It says nothing  
whatsoever about response to RH changes.
	Assuming the pattern I observed would repeat on controlled  
experiment, I draw the following conclusions:
1) The general consensus tends to be that if you take an in tune  
piano, after a temperature change has occurred, and once the plate has  
stabilized in temp, the piano will tend to be in tune with itself.  
This notion seems to be confirmed.
2) A piano will probably move slightly in pitch overall when the temp  
varies. But if room temp stays within parameters of, say, 60 to 80  
degrees or thereabouts (which probably expresses the extremes of what  
people are willing to put up with these days), the change in pitch is  
within a total of one hertz. Insignificant for most purposes.
3) If someone is worrying about storing a piano in a house over  
winter, where the temp will be allowed to approach freezing (assuming  
water in pipes, people will tend to maintain at least some heat),  
there is no reason to worry about the strings and plate. Even if there  
is no heat, and the piano is subjected to a change of 100F, the  
expectation is that the pitch change would be about 50 cents, hence no  
big worries about string breakage. (These questions have come up from  
time to time).

	BTW, we have moisture in the air, but it is seasonal. Now we are in  
the 5-15% RH season, with spikes to 25%. Late summer early fall, we  
are in 45% to 60% with some people managing to achieve 80% in their  
houses (by running an evaporative cooler constantly), with predictable  
results to tuning.
	I'll also note that my experience with harpsichords, which don't have  
a metal plate, are quite different. When they stabilize at a new  
temperature, they are considerably farther from initial pitch, and not  
in tune with themselves, with particularly acute differences between  
brass and steel wires.
Regards,
Fred Sturm
University of New Mexico
fssturm at unm.edu