Let’s first address the issue of definition. Piano tuning is a process of adjusting the relative pitch relationship of 88 different pitches. As with politics and religion – everyone has a different opinion as to what “in tune” means, even among professional piano tuners. To add to the difficulty of defining “in tune” is the inherent instability present in all pianos. Piano tuners who are honest about their work say they abandoned a tuning – not finished it. There comes a point in every tuning where further work doesn’t achieve any more noticeable improvement in the tuning. Further work at this point may actually decrease the stability of the tuning. This doesn’t mean that the piano is “in tune”. It just means that it can’t be made any more “in tune”.
Tuning is analogous to cleaning. If the room is very dirty, a preliminary cleaning is the first step to bring order and cleanliness to the room by removing clutter. A second and third cleaning may follow which may begin to clean the dirt and dust to that which is typical of a reasonably clean room. You can continue to clean away but it makes no appreciable difference to the overall cleanliness of the room. Does the dirt still present make the room “unclean”?
So what defines the “dirt” of tuning? In the simplest of terms, it is inappropriate “beats”. Beats or waves are generated by the interference between two pitches. These beats can be present at various speeds and between different notes. Tuning is a process of arriving at some optimized level of these beats.
You can sometimes hear beats within a single note. Play a note in the middle of the piano. Hold it down and listen for a wave or undulation in the pitch. If you hear any movement in the pitch, it is out of tune.
Play perfect 4ths and 5ths. Hold the notes and listen for the waves or beats. You should hear some movement between the notes. Generally, the 4ths will be ever so slightly faster than the 5ths. Their speeds though will depend upon their location on the keyboard.
How long a tuning lasts depends on the environmental stability, amount of pitch adjustment needed, condition of the instrument, and most importantly your standards. A piano can be tuned two or three times a day in a concert or recording setting. One time Sting rented one of our pianos for rehearsal purposes and I tuned it every morning for a week.
So the answer is, yes, your piano is out of tune right now, even if you had it tuned yesterday.