I have long had the impression that while a quick rise in RH (rain storm) leads to very rapid change in pitch in certain patterns, a fall in RH seems to have a slower effect. This past week I had a great opportunity to observe that.
Scenario: a series of five recording sessions, Sat through Wed, tuning before each. On Thursday before, I gave the piano a full tuning in preparation. RH was 19%, had been steady at 10 - 19% over the preceding month, had been tuned the week before. The tuning was simply a matter of polishing unisons on a stable instrument. This provided a base line.
Friday there was a short rain shower. I did not measure RH that day, but Sat when I tuned, it was 27%. Sun RH had dropped back to 18%, Mon had dropped to 12%, Tues back upward to 16%. Then there was another brief shower Tuesday afternoon. Wed RH was up to 26%. IOW, there was a rapid rise in RH, followed by just as rapid a fall, an additional fall, and then another rapid rise.
Results: Saturday the piano needed a complete tuning. There were fairly severe right/left string unison problems (right sharp, left flat or at pitch, especially G4 - G6, as I would expect from long experience), and the low tenor had risen significantly, with F2 3.5¢ sharp. I tuned to pitch throughout.
Sunday, the piano needed very little. Just unison polishing, I guess one could say that it had gone more flat than sharp, but on the order of 0.5¢. Monday, pretty much the same, but it was a wee bit flatter, and Tuesday again the same, now with F2 dropping 1¢. This morning (Wed), after the rain yesterday, I found F2 1¢ sharp and some unison issues with left string sharp, right flat pattern, less severe than on Saturday.
I conclude that wood absorbs moisture from the air more quickly than it releases it, in response to RH change - BUT keep in mind the actual readings, in the 10 - 30% range. I have no idea whether this would hold at higher RH levels. Practically speaking, it confirms what I had thought I had observed (but without day to day readings): if there is a rain storm, I definitely need to tune afterwards. But that tuning will hold fairly well for a good while.
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Fred Sturm
University of New Mexico
http://fredsturm.net "When I smell a flower, I don't think about how it was cultivated. I like to listen to music the same way." -Federico Mompou
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