By Taylor Mackinnon
The other important part of the piano is the action, which includes the keys, the intermediate power transfer portion of the action called the wippens, the hammers (hammer flange, hammer shank and hammer head) and the damper system that stops the sound after the hammers initiate the sound. We’ll cover the importance of the action later, but it’s helpful to realize that the action itself has eight to nine thousand pieces, most of which are hygroscopic, meaning they absorb and release moisture. This includes a lot of wood, felt, and leather.
What happens to a piano when there are changes in the humidity?
First the strung back. The sound board in a piano is analogous to the top of a guitar or a violin and, like those instruments, the sound board is bound to the superstructure or rim of the piano just around its perimeter so the sound board itself can move, vibrate, and create the sound. When the wood of the sound board begins to absorb moisture, it crowns up in the middle. Because the treble bridge is stretched across the middle of the soundboard, the strings get stretched since the bridges are attached to the board and therefore, the pitch is pulled a little higher. Conversely, when the humidity drops, the sound board shrinks and the pitch of the instrument will not only return to its original position, but often it will drop down a little lower. If an unused piano doesn’t get tuned on a semi-regular basis, over time (years) the pitch of the entire instrument can drop as much as a half step.
It is fair to say that humidity is constantly fluctuating. It changes from day to night, from day to day, from week to week and season to season. The amount it changes is controlled by factors like how cold it is outside, for instance (in the winter) and how much the windows are open (in the summer) and, for instance, when the heat is turned down at night or when we are out of the house. In the winter when the heat is on to keep the house warm, it also dries out the air. And clearly, when the windows are open in the summer time there is more humidity in the air. These are just the extreme changes.