Rhythm

Musical Rhythm has been defined in different ways and from different perspectives along the History of music theory. However, all the definitions agree that it is closely linked with the temporal dimension of music. A broad definition that could be applied in all the musical genres and styles is that rhythm is the organization of events in time.

Perception of Rhythm and Metrical Structure

The regularity of beats in tonal music allowed the emergence of different ways of understanding rhythm from hearing in this type of music.

One of the most applied ways of thinking about rhythm in tonal music analysis by means of perception has been linked with metrical structure. This is a hierarchical and multilevel structure of beats underlying musical sounds that determines the conditions of metric stability of sounds. Thus, the succession of isochronous beats forms a pulse, but pulses are not always explicit in music, they are a mental construction made from minimum signs that are presented in music, such as musical sounds. In this structure, one of the pulses has more relevance, because it helps listeners to configure and understand rhythm. That pulse has been called tactus, and it is usually the pulse that dancers use to make their steps and orchestra directors take to move their batons and communicate with musicians. Generally, tactus is among 70 and 120 beats per minute (bpm), but selection of tactus may vary depending on many factors, like listeners experience, type of music usually listened, instrument played, etc. From this basic level of beats the metrical structure is built mentally, with subordinate (faster) and superordinate (slower) pulses.

In points where the columns of beats are taller the stability of sounds will be higher, and vice versa. Another important issue to understand the metrical structure and its link with rhythm is the way that adjacent levels are related. Every level includes another level and/or is included in another level, and their relationship is only 2 by 1 (2 beats of the subordinate level by each one of the superordinate level) or 3 by 1 (3 beats of the subordinate level by each one of the superordinate level). Thus, the relationship among adjacent levels could be binary or ternary exclusively. To understand rhythm in a particular piece of music, the priority relationship is between tactus and subtactus (adjacent subordinated level to tactus).

References

Berger, A. (2002). The evolution of rhythmic notation. En T. Christensen (Ed.) Western music theory. Cambridge: Cambridge University Press.

Berry, W. (1967/87). Structural Functions in Music. New York: Dover.

Cooper, G y Meyer, L. B. (1960) The Rhythmic Structure of Music. Chicago: The University of Chicago Press.

Imberty, M. (1981). Les écritures du temps. Semantique psychologique de la musique. Tome 2. París : Dunod.
Lerdahl, F. y Jackendoff, R. (1983). A Generative Theory of Tonal Music. Cambridge, MA: The MIT Press.

London, J. (2002). Rhythm in twenty-century theory. En T. Christensen (Ed.) The Cambridge History of Western Music Theory. Cambridge: University Press, pp. 695-725.
Termperley, D. (2001). The cognition of Basic Musical Structures. Cambridge, MA: The MIT Press.