Your piano is designed to sound its best when tuned to
A-440 (A above middle C vibrates at 440 cycles per second), the international
pitch standard. At this pitch, power and tonal range are optimum and your piano
will match the pitch of other instruments. When your piano varies from A-440,
pitch adjustments are required to bring it back to standard. By always
maintaining your piano at standard pitch, you create long-term tuning stability
because the strings and structure stay in equilibrium. You also ensure proper
ear training because you always hear your music in the correct key.
Why does a piano's pitch change?
Piano strings change pitch for two primary reasons: the initial
stretching and settling of strings when the piano is new, and soundboard
movement due to humidity variation. In the case of new pianos, the pitch drops
quickly for the first couple of years as the new strings stretch and wood parts
settle. It's very important to maintain any new piano at the proper pitch during
this period, so the string tension and piano structure can reach a stable
equilibrium. (Most piano manufacturers recommend three to four tunings the first
year, and at least two per year after that.)
Aside from this initial settling, climate change is the main cause of pitch change. That's because the piano's main acoustical structure -- the soundboard -- is made of wood. While wooden soundboards produce a wonderful sound, they also react constantly to climate changes. As the relative humidity goes up, the soundboard swells, increasing its crowned shape and stretching the piano's strings to a higher pitch. Then during dry times the soundboard flattens out, lowering tension on the strings and causing the pitch to drop. The drop in the dry season tends to exceed the rise during humid times, so the net result is a drop in pitch each year that the piano isn't serviced.
If a piano has gone without tuning for an extended period, its pitch may have dropped far below A- 440. This means that each of its approximately 220 strings needs to be tightened considerably, adding tremendous additional tension to the piano's structure. The problem is that as each string is tightened, the additional load causes the pitch of previously adjusted strings to change. Thus it is impossible to make a substantial change in pitch and end up with a fine, accurate tuning in one step. Instead, a process called "pitch raising" must first be done, in which all strings are raised to their correct average tension levels. Only then can the piano be accurately tuned. In other words, accurate tuning is only possible when all strings are so close to their proper tension that only small further changes are needed during tuning. These small changes then do not disturb the tuning of other strings.
Like your car, your piano is a major investment which deserves regular servicing to keep it working well and preserve its value. Most importantly, the well-maintained piano sounds better, plays better, and gives you and your family a wealth of musical pleasure.