Unisons are comprised of the strings that make up the pitch of a note. When you play middle C for instance, there are 3 strings that are activated. To get the purest sound these strings need to be tuned exactly to each other. If they don’t match, there will be an unwanted beat. There are exceptions but always best to let the ear make the final judgement. The PTG Tuning exam checks for these beats by comparing the pitch of each of the strings to each other. After a one cent tolerance, points are deducted based on pitch discrepancy. This part of the exam is in the midrange from note 28-52. I believe that I tune unisons even closer than one cent. I have measured them once in a while and that seems to be true.
Tuning the Temperament
I tune with either Equal Temperament or slight modification on that one which I call the “Vibrato Temperament”. I have been using the latter for about 8 years and that is my preference. With the Vibrato Temperament intervals don’t gradually speed up as with Equal Temperament. Beat rates are set in a temperament octave of your choice. Usually that octave is A octave 3 to A octave 4 or as I do it F octave 3 to F octave 4 and then spread out via octave tuning to the rest of the piano. The notes in an Equal Temperament octave don’t have much attention given to them aside from having them speed up evenly and to fit within the temperament octave. . With Vibrato Temp. the beat rates of intervals can be slowed down or sped up as compared to Equal Temp. A lot can be written about octave tuning which takes skill regardless of one’s temperament choice. Having been a violinist-violist earlier in life, I realize now that Major 3rds and 6ths for example are played pure on violin, etc. so that there are no beat rates to worry about. Of course you still need to play them pure which is a challenge. On the piano you can hear faster beat rates if you learn to listen to them. If one tried to tune a piano with pure 3rds and 6ths, the piano would sound awful. F to A octave 3 is set at about 6 beats per sec. in Vibrato Temp., but it varies according to the piano. With Equal Temp. it is closer 7 beats per sec. That is too fast for my ear on most pianos. I need to include a audio example hear of some differences on a real piano.
Temperaments are kind of like fonts in word processing. You can design your own font and then apply the same characteristics to all the letters of the alphabet or you select fonts that someone else designed. The same applies to temperaments except that a temperament is determined by 12 notes and then they are extended out by octaves to the rest of the keyboard.
There are different approaches to treble tuning. Some tuners stretch the top 2 octaves more than others. A 4:1 octave is one way of tuning those octaves. That type octave gives some brightness without distracting beats. A 2:1 octave sounds in tune when playing single octaves, but with double octaves the top note could sound flat to some. The flatness is very noticeable when striking a note 2 octaves down by itself and then hitting the top note. There is a tendency for the human ear to hear the top note lower than when playing octaves.
Tuning the Bass
There are some special challenges in tuning this section of the piano. Since all the beats that a tuner listens to are occurring at a slower rate then in the midrange and the treble, one has to take some additional time. Otherwise unisons can be left out of tune. Unisons are the strings that make up a note; there can be 1, 2 or 3 strings per note in the bass. The challenge is in getting them all to match. As in the treble there are different ways to tune bass octaves. In order to tell which approach works best in the bass, I play 3 octaves at the same time using both hands of course. I try to get the best blend of the various partials. I also use either an Sanderson Accutuner 2 or 3. I use partial matching. You can often match 8:4 and 6:3 octaves in the low bass and sometimes even in the upper bass. If you want a wider octave, there is 12:6 partials. On small pianos like a spinet or console you don’t need a really wide octave. Best to check everything by ear.
Creating a stable tuning is an art which takes years to achieve. It comes from skill in using the tuning hammer or lever and also in how the key is struck. I tend to strike the key with a hard blow rather than a light one.