​​Your piano is made primarily of wood, a versatile and beautiful material ideal for piano construction — but greatly affected by humidity.  Seasonal and even daily changes in humidity cause wood parts to swell and shrink, affecting tuning stability and touch.  Extreme swings in humidity can eventually cause wood to crack and glue joints to fail, even the finish to crack.

Other materials in your piano are also affected by changes in moisture content in the air.  The many felt and leather parts in your piano’s action can change dimension, affecting regulation and friction, or stiffness of the touch.  Very high humidity can even create condensation on metal parts such as strings, tuning pins, and hardware, eventually causing them to rust.

As the moisture level in the soundboard increases during periods of high relative humidity, the crown expands and pushes the bridge harder against the strings.  The strings are stretched more tightly and the piano’s pitch rises.  Because this increase in crown is greater in the center of the soundboard than at the edges, the pitch rises more in the middle octaves than in the bass or treble registers.

During periods of low relative humidity, the soundboard shrinks, reducing the crown and decreasing pressure against the strings.  The pitch drops, again with the greatest effect noticeable in the center of the keyboard.  When relative humidity returns to its previous level, the average pitch of all the strings will return to normal, although the exact pitch of individual strings will be slightly changed from their original settings.

Thus, a piano only will stay in tune as long as the relative humidity level in the air surrounding the soundboard remains constant.  Extreme humidity changes require making greater changes in string tension to bring the piano into tune.  This upsets the equilibrium between the string tension and the piano frame, and the piano never becomes stable.

Keeping the humidity level around your piano as constant as possible will help it stay in tune longer, as well as slow such damage as soundboard cracks, loose tuning pins, and glue-joint failures.  Some simple things you can do:

• Position your piano away from areas where it would be exposed to extremes of temperature and humidity such as heating and cooling vents, stoves, doors, and windows.

• Direct sunlight is especially damaging.  Try to avoid it.

• If your home is not well insulated, an interior wall is preferable to an outside wall.

There are a variety of electronic-controlled dehumidifier products available to fit all sizes of uprights and grands.  Ask your piano technician to advise you of the best choices for your unique circumstances.