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In music theory, the spiral array model is an extended type of pitch space. A mathematical model involving concentric helices , it represents human perceptions of pitches, chords, and keys in the same geometric space. It was proposed in 2000 by Elaine Chew in her MIT doctoral thesis Toward a Mathematical Model of Tonality. Further research by Chew and others have produced modifications of the spiral array model, and, applied it to various problems in music theory and practice, such as key finding , pitch spelling, tonal segmentation, similarity assessment, and musical humor. The extensions and applications are described in Mathematical and Computational Modeling of Tonality: Theory and Applications.
The spiral array model can be viewed as a generalized tonnetz, which maps pitches into a two-dimensional lattice structure. The spiral array wraps up the two-dimensional tonnetz into a three-dimensional lattice, and models higher order structures such as chords and keys in the interior of the lattice space. This allows the spiral array model to produce geometric interpretations of relationships between low- and high-level structures. For example, it is possible to model and measure geometrically the distance between a particular pitch and a particular key, both represented as points in the spiral array space. To preserve pitch spelling, because musically A# ≠ Bb in their function and usage, the spiral array does not assume enharmonic equivalence, i.e. it does not fold into a torus. The spatial relationships between pitches, between chords, and between keys agree with those in other representations of tonal space.
The model and its real-time algorithms have been implemented in the tonal visualization software MuSA.RT and a free app, MuSA_RT, both of which have been used in music education videos and in live performance.