Aside from this initial settling, seasonal change is the primary reason pianos go out of tune. To understand why, you must realize that the piano’s main acoustical structure, the soundboard, is made of wood (typically 3/8-inch thick Sitka spruce). And while the wooden soundboards produce a wonderful sound, they also react constantly to weather. As humidity goes up, a soundboard swells, increasing its crowned shape and stretching the piano’s strings to a higher pitch. During dry times, the soundboard flattens out, lowering tension on the strings and causing the pitch to drop.
Unfortunately, the strings don’t change pitch equally. Those near the soundboard’s edge move the least, and those near the center move the most. So unless it’s in a hermetically sealed chamber, every piano is constantly going out of tune!
The good news is that there are some simple things you can do to keep your piano sounding sweet and harmonious between regular service appointments. Although it’s impossible to prevent every minor variation in indoor climate, you can often improve conditions for your piano.
} Locate the piano away from direct sunlight, drafts, and heat sources. Excess heating causes extreme dryness, so try to keep the temperature moderate (below 70 degrees) during the winter heating season.
} Get a portable room humidifier, or install a central humidification system to combat winter dryness in climates with very cold, dry winters. A portable dehumidifier or a dehumidifier added to your air-conditioning system can remove excess moisture during hot, muggy summers. The goal is to even out the humidity, so the seasonal changes are less extreme.
} If controlling your home’s environment is impractical, or if you want the best protection possible, have a humidity control system installed inside your piano. These are very effective in controlling the climate within the instrument itself. Besides improving tuning stability, they help minimize the constant swelling and shrinking of your piano’s wooden parts. The critical part of such a system is the humidistat, a device that monitors the relative humidity within the piano and adds or removes moisture as needed. Jars of water, light bulbs, or other “home remedies” have no such control and can actually do more harm than good.