The Perceptual Tuning Primer
Perceptual Tuning refers to arranging TIA music for particular keys, with the intent to shift off-key notes to ones your brain won't as readily detect as being off-key.
This section is rather heavy, while using Perceptual Tuning is rather simple in practice. If you find yourself overwhelmed by the theory, feel free to skip ahead to the Practical Application section.
When a note sounds "off" to a listener, it isn't because the listener hears that the note is 262 Hz rather than 261.626Hz. It's because the frequency relationship of the note sounds off in relation to other notes in the music. If you speed up the playback of a recording of Beethoven's 5th symphony, the pitch of the notes being played are shifted upwards, but overall the recording will still sound in tune because the relationships between those notes are remain the same.
Our brains are better at picking out bad tuning for some note relationships better than others. When 2 or more note frequencies line up with regular periodicity, our brains detect that. There's even have a musical name for the effect - it's called consonance. When the frequencies don't line up regularly, we call that dissonance.
[diagram illustrating frequency dissonance vs consonance]
When we play 2 or more notes that are normally consonant on an instrument that is slightly out of tune, our brains easily pick out the loss of consonance. When we play 2 or more notes together that are normally dissonant on the same out-of-tune instrument, the result is still dissonance. Because of this, our brains are less sensitive to bad tuning for dissonant notes, then they are to bad tuning for consonant notes.
The last bit of theory we need to understand is that almost all music is written in a particular key, which can be thought of as a foundation note - the notes played in the song are interpreted by our brains in relation to the key, just as if they had been played at the same time as the key. As the music unfolds, the different notes being played vary in pitch, but they also vary the level of consonance and dissonance relative to the key. As you might expect, if any given note is consonant to the key, our brains are more sensitive to that note being out of tune.
While TIA is incapable of creating music that is in absolutely in tune, we can pick a key for creating TIA music that will ensure that consonant notes are closer to being in tune, leaving the out-of-tune notes to dissonant notes. This is what we call Perceptual Tuning.
To find the key that had the best TIA Perceptual Tuning properties, a program was created that ran through all of the available frequencies and scored them according to how in-tune the following notes were. The score for any given note was weighted according to the relationship of the note to it's key, with much larger weights being assigned to notes with strong consonance - namely the major third, the perfect fifth, and octave.
The program found that A3@218.3Hz had the best Perceptual Tuning score, with the first 12 semitones having good perceptual tuning, with the next 12 semitones being decent except for an out-of-tune major third at Cfirstname.lastname@example.orgHz.
The musical key with the best TIA Perceptual Tuning for any 12 consecutive notes is A3@218.3Hz, and the next higher 12 notes is good as well if you can avoid the major third at Cemail@example.comHz. Write your songs in the key of A3, and you'll take advantage of Perceptual Tuning.
There's a lot of theory behind it, but in practice Perceptual Tuning is very simple to implement. Check out the samples below to hear the difference between regular tuning, tune2600 selected tuning, and perceptual tuning. The sample represent a worst case example: a familiar song that makes extensive use of sustained chords.