I have a number of thermo-hygrographs here at the Conservatorium. These are barrel-type recorders which produce a graph of temp and relative humidity in the air over a week. It is clear from the graphs that a rise in temperature produces a fall in humidity and vice versa in an air conditioned environment ( with no humidity control ). The amount of humidity change relative to temperature change varies - at humidity levels of 50-60% there is a greater effect from one degree of temperature change than at humidity levels of 70-80%. As an average, a one degree ( Celsius ) change in temperature produces an 8-10% change in humidity The actual effect on a tuning is made more complex by the ' lag' in moisture release by timber mentioned by Michael Jorgensen , variations in the humidity levels of the outside air being sourced by the air conditioning plant day time/night time, the position of the instrument and so on. What is clear to me after observing these figures and their effect on the pianos I care for is: - day/night changes in temperature produce changes in humidity levels which affect tuning in the same way as if the piano was moved into different humidity environments. Simply, if humidity rises then the soundboard will seek to absorb that humidity, and the pitch will rise, especially in the middle register; and generally unisons will become unstable. - the worst effect is due to a spike, where temperature rises then falls a few degrees over a short period. A piano in that environment 'feels' humidity levels move 10% then return 10% over the time - effectively +20% change. This is the scenario you are describing, John. No amount of good hammer technique can secure a tuning against the tension changes that the corresponding board movements will introduces into capo, front and rear duplex string segments. These pianos sound appalling - but take very little time to rectify. Ask me how I know! However they can do your reputation a lot of harm in a short while. Talk your customer through these facts Geoffrey Pollard Piano Technician Sydney Conservatorium of Music > > >Anyone have any hard evidence(or direct experience) with tuning >>instability caused by regular temperature fluctuations? Several private >>customers are in the habit of turning the thermostat on 60 degrees F at >>bedtime. (brrrrrr) Daytime temps are 68-70ish. Would this wreak havoc on >>tuning? >> >>John Minor > >University of Illinois > > >>
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