In musical terms, rhythm refers to the basic pulsation of the music, how the music is organised in time and how it is emphasised. Rhythm becomes vivid and significant when its beats are given different emphases. We all know how rhythm works in speaking: a word that has an unusual rhythm or stress can be difficult to understand. A good sense of rhythm has been scientifically associated with literacy and language learning.
Rhythm in the Renaissance era
In Renaissance music, rhythm is closely tied to polyphony, which means many voices sounding at once. When several melodies are superimposed, the various voices sometimes each have their own rhythm. Therefore it is not always possible to detect a clear rhythm in this music, something to which you could count three or four beats in a bar. Instead of rhythm, there is a common pulse according to which all the voices move.
Rhythm in the Baroque era
During the Baroque era, bars or measures became the basic unit of musical rhythm. In Baroque music, it is possible to count along with the music, with three or four beats in a bar. The solid foundation of a Baroque orchestra is the basso continuo group, which governs the rhythm and harmony of the ensemble.
Rhythm in the Classical era
Abandoning the basso continuo and writing separate parts for bass and harmonic lines changed the nature of orchestral accompaniment. The aim now was for a systematic, natural and simple musical expression. The writing of individual parts to form the accompaniment allowed a more precise and detailed treatment of rhythm. In orchestral works, this can be perceived as a rhythmic framework in the middle voices under which the bass line progresses. Harmony was built into this rhythmic pulse, between the melody and the bass line.
Rhythm in the Romantic era
Rhythms became more complex, and extremities of speed were explored. Slow became really slow, fast became really fast. Improved musical training and the emergence of virtuoso soloists led to composers writing faster and rhythmically more complex music.
Rhythm in Impressionism
In Impressionist music, rhythm became more complex. A rhythmic motif might be used to create a fluid texture made up of repeated rhythmic fragments where the pulse may be obscured.
Rhythm in Expressionism
The liberation of form and harmony also affected rhythm, which became more complex, irregular and often shifting in tempo.
Rhythm in the modern era
Technology and a long tradition of performing complicated rhythms enable both minimalist repetitive structures and complex rhythmic landscapes. Serialism, randomness and polyrhythms are integral elements in music. Rhythm may also be indeterminate, as in the case of noise.