Old and New Isometries between Pc sets in the Planet-4D Model
2015, Music Theory Online
https://doi.org/10.30535/MTO.21.3.1Abstract
The Planet-4D model introduced by Baroin 2011b is a richer model of pitch-class relationships than the standard cyclic model of the clock-face diagram: instead of the distribution of twelve points on a circle in a 2D plane, the Planet-4D model places the twelve pcs on a 4D hypersphere. Beyond the usual T/I group of symmetries, which is still featured in this new musical space, the additional dimensions yield other isometries (most notably now including the M5/M7 operations), which appear here in animated visualizations of musical pieces. The present article elucidates how these isometries are in fact well-known symmetries that do not preserve shape in most previous models with fewer dimensions. The hyper-spherical environment grants each symbol an equivalent physical position, and therefore allows for more symmetries than any 3D model. This article also shows that the model provides a visually intuitive geometric setting of pcs, in accordance with our perception of 3D Euclidean spac...
References (13)
- Major or minor triads provide a helpful demonstration of this larger group: the original T/I group acts simply transitively on these 24 triads; in other words, there is exactly one way to transform one given triad into another one by way of a T/I transformation. This is one of the most famous generalized interval systems as defined in Lewin 1987.
- Applying any of the new isometries, however, turns these triads into the so-called major-minor trichords, i.e., transpositions of 014 or 034, set class 3-3 in Forte's classification. (In Fiore, Noll, and Satyendra 2013 these chords are called "jets" and "sharks" respectively, in honor of their use in the score of Bernstein's West Side Story.) One example of such a transformation was given above in [21.3]. To give another example, the transformation turns the pitches of the C-major triad, C = (0, 0), E = (1, 0), and G = (1, 1), into the "jet" chord B = (2, 3), C = (0, 0), and E = (0,3) respectively; see Video 10 for a representation that shows [0,4,7] transformed into [11,0,3]. Of course the tonal quality of the triad is destroyed, as is the intervallic content in -see, for example, what becomes of the aforementioned sequence of triads in Beethoven's Ninth Symphony when this transformation is applied in Video 11.
- Some useful features of the new group are given in Proposition 2. Theorem 2, below, explains why they may look familiar to many readers.
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