I am trying to interpret the fingering position chart (page 112 of Shamisen of Japan, 3rd Edition) in terms of pitch intervals. Since the position numbers omit some of the semitones in the western chromatic scale, I’m not certain what pitch I should expect to hear, especially on the upper notes.
The lower (i.e. closer to the kamigoma) fingering positions are fairly easy to figure out by ear. At the upper end, the positions get pretty close together. Position four (at the outer neck splice) corresponds to a perfect fourth interval. Somewhere else I read that position 10 (39.84 cm) represents the octave, which suggests that the chart represents an open string length of 79.68 cm. Given that frequency is inversely proportional to the vibrating string length, the second octave position should be 59.76 cm from the kamigoma. On the chart, that lands between position 20 (58.73 cm) and 21 (60 cm).
Following this thought, I translated the fingering distances in the chart into frequencies (for a given string pitch, e.g. “C”), and compared them to an “equal tempered” scale. The results were rather inconsistent; some intervals were higher, and some were lower. This would be expected if the positions represented a temperament other than “equal”, but the variations were not consistent between the lower and upper octaves. For example, the fifth interval was high in the lower octave (position 6), and low in the upper octave (position 16). Also, while positions # and 4 correspond reasonably well to a major third and a perfect fourth, position 14 falls halfway between the two intervals. And, as noted above, is the second octave supposed to be position 20 or 21?
So, my question is, does this chart represent non-Western intervals, a non-equal temperament, or are the locations simply incorrect?