Tonal Hierarchies in Jazz Improvisation

Statistical methods were used to investigate 18 bebop-styled jazz improvisations based on the so- called Rhythm Changes chord progression. The data were compared with results obtained by C. L. Krumhansl and her colleagues in empirical tests investigating the perceived stability of the tones in the chromatic scale in various contexts. Comparisons were also made with data on the statistical distribution of the 12 chromatic tones in actual European art music. It was found that the chorus- level hierarchies (measured over a whole chorus) are remarkably similar to the rating profiles obtained in empirical tests and to the relative frequencies of the tones in European art music. The chord- level hierarchies (measured over single chords) suggest that in the chord progression the improvisers have certain cognitive reference points that are outlined more carefully than the rest of the chords (strong local hierarchy vs. weak local hierarchy). Furthermore, in both analyses, it was found that the metrical structure was used to emphasize or de- emphasize tones, depending on their tonal function.