Occasionally, I have clients ask me: If my piano is not being played, do I need to have it tuned? The short answer is YES. Most people believe that if a piano is not being played, it will not go out of tune. Surprisingly, moderate playing of a piano is usually NOT the primary cause of it going out of tune. More often changes in humidity and temperature are the real culprit. Of course heavy playing for several hours a day will cause any piano to go out of tune, but most piano owners are not using their instruments in this way.
For many, a piano goes out of tune simply because the inside environment of their home changes with the seasons, not because of usage. Keeping your piano tuned regardless of usage is recommended to maintain the stability and equilibrium of your piano.
The strings on your piano exert a significant amount of downward pressure on the largest component in your instrument – the soundboard. Because the soundboard is made of wood, it reacts to the amount of humidity present in the surrounding environment. When humidity is high, it will absorb moisture causing it to bend in one direction. Conversely, when the humidity level is low, it will bend in the opposite direction, as it releases the moisure. This back and forth movement of the soundboard, causes the strings to become sharp or flat and eventually out-of-tune.
Some pianos stay in tune a long time, while others do not. Although there are many reason for this, changes in humidity are most often the real cause.