A piano, because of its construction, is always under a varying amount of stress. The 232+ odd strings are stretched at an average tension of one hundred fifty to two hundred pounds apiece, so that the iron plate and the heavy wooden frame carry a strain from eighteen to twenty tons! Since the steel wires are elastic, and the soundboard is a thin sheet of spruce wood, the stress is not constant. This elasticity in the soundboard is necessary in order for the piano to produce a pleasant tone.
So what is the major factor involved in a piano going out of tune? Humidity seems to be the main culprit for causing variations in tuning. As the humidity increases, the wood of the soundboard swells causing the crown of the soundboard to rise, thus raising the pitch. During the colder months when the humidity drops, a drying out of the air occurs. The crown contracts and the pitch is lowered. this ‘rising and falling’ of the soundboard usually results in a net effect of the piano going flat. This is one of the reasons why pianos should be tuned & maintained continuously. I often hear the phrase, “we hardly ever play it.”
Rarely are pianos found to be sharp except in extreme humidity conditions. Anything above A440 is considered sharp or high in pitch. When tuning a piano, each note between the octaves are ‘equally tempered’ by the piano tuner so that chords sound pleasant to the ear.
There are other factors that can cause pianos to be out of tune such as: loose tuning pins, faulty bridges, loose stagger pins, and other factors.
Since there are seasonal variations in every part of the world, pianos, no matter how well they are constructed, need to be tuned. To not do so is to neglect and possibly destroy a fine piece of craftsmanship. Since pianos are built to withstand a certain amount of tension, it follows that they need to be maintained at that tension.
It is recommended that a piano be tuned every six months not to exceed one year regardless of how much the instrument is played. This will help to ensure that you get accustomed to the correct pitch, and that you experience maximum enjoyment while playing your instrument.