Many of us notice that with seasonal changes, our piano also changes. Maybe we hear that the piano, which had sounded really crisp and clean just a week ago, is sounding sour. Maybe the octaves or the fifths are starting to sound out of tune. Perhaps the middle of the piano sounds discordant when played with the bass or the treble.
The most likely cause of these changes in your piano are changes in the relative humidity of the air around the piano.
Piano soundboards are relatively thin panels of high quality solid wood. Typically manufacturers use the best pieces of spruce that they can find. Although soundboards look flat, they actually have a bit of a curve, or "crown." It's important for soundboards to have this slight curvature in order to vibrate effectively and produce good tone.
The piano's strings press down on the bridge, which is a solid piece of wood that is approximately an inch thick and around an inch and a half in width. When a piano string vibrates, the vibrational energy is transmitted through the bridge into the soundboard, which causes the soundboard to vibrate and amplify the string's sound.
So, why do pianos tend to go out of tune with seasonal changes? It's because the soundboard is changing along with the seasons. In the warmer months of the year, there is more moisture in the air. The wood cells in the soundboard absorb the moisture and swell up. With the soundboard's wood cells getting larger, the soundboard's "crown," or curvature, is enhanced. The soundboard presses up with more force against the bridge, especially in the middle of the soundboard, which corresponds to the middle octaves of the piano. The increased tension created by the soundboard against the bridge causes increased tension in the strings, especially in the middle octaves, which causes the pitch to rise. Because the soundboard moves up with more tension in the middle of the piano, the middle octaves tend to go sharper than the octaves in the bass and treble. The piano starts to sound out of tune with itself.
In the colder months of the year, the air holds less moisture. Relative humidity goes down with cooler temperatures. And when we heat our homes, in many home environments the heating system tends to further dry out the air. As a result, the wood cells in the piano soundboard release moisture. The cells shrink, and so the soundboard goes down, reducing tension on the bridge. Again, the biggest change is in the middle octaves of the piano, because that's where the soundboard is moving the most. And, the middle octaves of the piano go down in pitch, more than in the bass or treble. And the piano again goes out of tune with itself.
All of these seasonal dynamics are even more pronounced in large facilities, such as schools, auditoriums, and houses of worship. Powerful HVAC systems circulate air with great efficiency, but most don't have very robust humidity control systems in place. The relatively fast circulation of air accelerates the changes in relative humidity, and the piano soundboard gains or loses moisture more rapidly. As a result, the piano tends to go out of tune more rapidly, especially in a large facility when beginning to heat air in the fall, and when ending heating in the spring.
Piano tuners and technicians do our best to provide tuning and repair services, so that you may enjoy your instrument to the fullest extent. It's important to understand that no tuner has a magic tuning that avoids or eliminates the changes that occur with humidity changes. There are humidity control solutions that help to lessen changes in humidity, especially in the spaces immediately surrounding the soundboard, and these systems, when correctly installed and properly maintained, do help to lessen seasonal changes.
I am a certified installer of the Piano Life Saver (Dampp-Chaser) humidity control system. I have installed one in my piano at home and in the pianos of some of my clients. The system helps to lessen the effects of seasonal humidity changes. I would be happy to work with you to develop the best solution for you and your piano. Thank you.