Tuning is the job most often requested of a technician. But it is often the most misunderstood job of fine piano preparation. A very fine quality tuning can actually sound poorly if the piano is poorly voiced. Also, a very fine quality tuning will not last more than a few hours if the piano is not tuned frequently enough. This is because over time the piano goes out of tune in two ways . . . 1. to itself (so the intervals and harmonies don't sound right anymore), and 2. the entire pitch of the piano drops away from the standard of A=440.*
Piano string wire is made of high carbon spring steel and consequently it has all the characteristics of any spring, i.e. when you pull a spring apart and let go of it, it will pull itself back to its "homeostatic tension point." Conversely, when you push a spring together and let go of it, it will push itself back out to it's homeostatic tension point. Piano wire has the same characteristic. When we pull it tighter it will try to pull back, and when we loosen it, it will actually try to push back up (tighter). The internal tension in the string will increase when we pull or push it away from its homeostatic tension point. The further we have to pull a string to get it to reach proper pitch, the stronger the pull back becomes (just like in a spring) We can all bend a leaf spring, or pull out a coil spring a little bit. But if it is a strong/heavy spring we can't move it very far. We notice that it gets harder and harder to pull out until we just can't pull it (or compress) it any more. These is because the "pull back" or "push back" force increases greatly the further we try to move it away from its homeostatic tension point. The same thing happens with piano wire so that if a tuner has to move it too far away from its current homeostatic tension point the pull (or push) back become so great that the string itself will overcome some of the new setting and literally pull (or push) itself back "out of tune" some amount. It will not stay exactly at the pitch the tuner moved it to. The further the tuner has to move the string way from its current homeostatic tension point to get it to the proper pitch, the strong and more pronounced will be the string pulling itself back out of tune. This effect is immediate and can be easily hear within a few minutes.
Now here's the reason for sufficiently frequent tunings. Over time the internal homeostatic tension point in the piano wire relaxes and falls further and further away from the A=440 standard. Thus, the tuner needs to do two things when tuning: 1. bring the pitch back to the 440 standard and, 2. insure the piano is all in fine relational harmony to itself. But if it has been too long since the last couple tunings, the homeostatic tension has fallen too far away from 440 to allow it to be brought back to that point without the pullback being too strong, thus preventing the piano pins from keeping it at the pitch where the tuner set it. In these cases, the tuner will need to "raise the pitch" then "tune" the piano. He will need to follow that up with another "final" tuning within a few weeks (2-4) to be able to get the piano wire to accept a new homeostatic tension point that is now at the 440 standard. At this point, the whole matter now works in favor of the piano owner, because now the piano wire will resist moving away from its new homeostatic tension point, which is now exactly at the International pitch standard of A=440.
To keep a piano from falling too far away from the 440 standard so that a very fine tuning can be put on it and the wire will allow the change most home pianos should be tuned twice a year here in Eastern WA.
*A=440. This simply means that the sound wave on the first "A" note above Middle "C" will cycle 440 times each second. At that speed, we will hear the correct pitch. If the string is pulled too tight, the sound wave will cycle more than 440 times per second and we will hear it as a higher pitch. Conversely, if we do not pull the string adequately tight, the sound wave it produced when struck will cycle slower than 440 times per second and consequently we will hear it as a lower pitch.