What makes a piano go out of tune?
Changes in humidity and temperature as well as use and time all affect tuning stability.
Pianos go out of tune primarily because of changes in humidity. Pianos are mostly made of wood. One characteristic of wood is that it absorbs moisture from the air or releases moisture into the air until an equilibrium is reached. When wood absorbs moisture from humid air it expands. It actually changes dimension and gets bigger. When wood releases moisture into dry air, it gets smaller. You have likely noticed this change if you have ever observed wood floors through the seasons. In the winter, when the air is dry from heating, the wooden slats get slightly smaller. What we see is that the cracks between the wooden slats grow bigger. A gap appears. The opposite is true in the summer. As the slats expand with moisture, the gap disappears or grows smaller. It is this same phenomenon that causes doors or windows to stick or bind in the summer, but then free up in the winter.
These same changes in the dimensions of wood in your piano cause it to go out of tune. In particular, it is changes in the wooden soundboard that cause a piano’s pitch level to change. When your piano is exposed to the dry air in winter, your piano goes flat. The pitch of the strings gets lower. The opposite is true in the summer, when moist humid air causes the pitch level to rise. Your piano goes sharp. These changes in pitch don’t happen evenly throughout the piano. They tend to be most pronounced in the tenor and lower treble. Since these changes don’t happen evenly, we perceive the piano as out of tune.
The humidity level in your home changes most dramatically because of heating. In the winter, your furnace heats the air and dries it out. When you first begin heating in the late fall or early winter, there is a sudden dramatic change in the humidity level in your home. Correspondingly, the tuning of your piano will change noticeably during this transition into the winter heating season, as your piano goes flat. Again, in the spring when you stop heating, the air in your home is no longer being artificially dried by your furnace. Then the humidity level will rise and again the tuning of your piano will noticeably change, as your piano goes sharp. So your piano will go through two distinct cycles each year, with dramatic changes in the tuning of your piano taking place at the onset and end of the heating season.
Use, temperature, and time also affect tuning stability. Heavy playing can cause changes in the tuning, however, the better the tuner and the more frequently a piano is tuned, the less playing will cause changes in the tuning. Temperature changes also cause the pitch level of a piano to change, however, with modest changes this usually happens relatively evenly across the piano, so the relative tuning may not be significantly affected. Temperature changes during a tuning are problematic and a piano should be tuned at the same temperature at which it will be used. Finally, time has a gradual effect on tuning stability. Piano strings are at a very high tension. That tension represents potential energy which wants to dissipate. As it dissipates over time, the pitch level drops and the piano goes out of tune. So, even without use and in a stable environment, pianos should be serviced with some regularity.