Mostly you’ll be told once a year. Why?
There are a number of factors that cause your piano to go “out of tune”. Most of these are variable, but there is one overriding factor that is quite amazing: tension.
All piano strings are pulled to a very high tension. The strings of your piano are pulled so tight that when your piano is in tune, the strings are stretched to around 80% of their breaking strain. That’s a lot of tension! collectively it can add up to 20 Tonne.
It takes a cast iron frame to hold this pressure, which is why your piano is so heavy.
Over time, the piano will lose some of this tension, because it just cannot hold it indefinitely. With less tension on the string, the pitch drops, so the piano will not be “in tune” or “on concert pitch”.
A year is a good time to correct this pitch loss, and still have the tuning remain stable. When the piano is not tuned for a number of years, the amount of tension that needs to be added across the instrument can cause movement in the structure itself. This means a second or third tuning might be needed to safely add enough tension to get the piano’s pitch where it should be.
Tuning Pins: The tuning pins used to adjust each piano string’s tension must be held very firm, but still be movable. They are driven into the wood of the tuning plank very tightly. This is the reason special tools are required to tune the piano. It’s not possible to adjust the tuning pins with your fingers as you would with a violin or a guitar, which are much lower-tensioned instruments. Getting the feel of using these piano tuning tools accurately is something that takes years of practice, and it’s one of the reasons to hire a professional piano tuner.
Other Factors that effect Piano Tuning