Bridges: Mathematical Connections in Art, Music, and Science (2004)

Pages 189–194

A musical rhythm pattern is a sequence of note onsets. We consider repeating rhythm patterns, called rhythm cycles. Many typical rhythm cycles from Africa are asymmetric, meaning that they cannot be broken into two parts of equal duration. More precisely: if a rhythm cycle has a period of 2n beats, it is asymmetric if positions x and x + n do not both contain a note onset. We ask the questions (1) How many asymmetric rhythm cycles of period 2n are there? (2) Of these, how many have exactly r notes? We use Burnside's Lemma to count these rhythms. Our methods can also answer analogous questions involving division of rhythm cycles of length ln into l equal parts. Asymmetric rhythms may be used to construct rhythmic tiling canons, in the sense of Andreatta (2003).

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