In a good recent demonstration, Jonathan Bellman suggested that sequential passages in Chopin’s piano music have greater content with less equally tempered tuning. The specific pitch proportions within intervals would have subtly changed as key-area changed, with the less equalized tuning of 19th-century pianos.
Equal temperament remains a theory but is not a practical piano reality even today. Just listen…
Technicians in my school can recognize the tuning done by a colleague. “Zoe was here!” In contemporary American practice, many piano technicians start by setting a central octave of pitches from F to F, or perhaps from A to A. To some, the differing results are audible.
Accustomed to the stretched upper pitches in the highest octave of keys on Steinways tuned by the Steinway Concert and Artist department in New York (tuning the notes higher makes for brilliance), I was surprised by the pitches I heard as I played the highest notes in London — when I began performing there. At Steinway in Marylebone Lane, Robert Glazebrook probably smiled as he told me “We don’t stretch in London.”
A fifth not tuned in exact 3:2 proportion will cause subtle “beating” — an audible rhythmic pulsation — between the notes forming the interval. On modern pianos, all intervals except octaves are detuned slightly. In a slow, gradually developing piece with a lot of repetition, like Alvin Curran’s Inner Cities II, I’m sure the specific tuning, and the ensuing beating that occurs between notes, influences the pace and rhythmic delivery of the music. Sometimes beating makes it hard to play notes that are very close together (chromatic half-steps) in a way so their attacks seem simultaneous.
Ian Pace has described how a pianist might distinguish the pitch A-flat from G-sharp in piano music by Morton Feldman. The intensity or weighting of other pitches being played has an effect on our perception of pitch, and our perception of what’s coming next. (Generally in notation, after a G-sharp comes an A. While an A-flat is followed by G.)
After the Chopin demonstration, I realized pianists still use the expressive possibility of slight variations in interval tuning. Unlike string players who might expressively shade the exact sharpness or flatness of a pitch, attentive pianists will be responders. Pushing a bit more on one key or another or waiting infinitesimally more or less, we react to the tuning that the instrument has, and to the microscopic shifts that occur as pitches flatten in concerts. (One notable exception I recall being a brightly stage-lit instrument that seemed to get sharper as the heat of the lights kept radiating onto the sound board.)
One note “out of tune” — often one of its three strings no longer matches the others well — can be somewhat ameliorated by different weightings of notes in the overall texture. These are not conscious adjustments so much as physical responses controlled by listening.