Rhythm

In general, pattern in space and time. With sound, rhythm describes the pattern of events in time, or as Ezra Pound put it, "Rhythm is form cut into time."

The speed of rhythm is its tempo. The range of tempi includes those much slower than those of human experience, called infra-biological, and those much faster, called suprabiological. In between lies a narrow range of speeds common to human activity. Many developments of technology have introduced faster and more complex rhythms into the soundscape, including those where separate events can no longer be discerned (see drone, stationary sound).

A large variety of rhythms are based on a cyclic or periodic repetition of events (see diagrams under cycle and tempo). The regularity of cyclic rhythm is to be found in such patterns as the inhalation and exhalation of breath, the systole and diastole of pulse, the ebb and flow of tides. The period of repetition is sometimes called the measure or bar, as it is in music. Metric rhythm, which is common to many types of music, involves the subdivision of the measure into equal units marked by a pulse called the beat. The first beat of the measure is usually accented.

See: Metronome, Note, Polyphony.

Metric rhythm may be isometric or multimetric. In both cases, all time values are fractions or multiples of a beat; but in isometric rhythm, the groups of beats or measures are equal, with the first beat usually accented. In multimetric rhythm there is no regular grouping of beats and hence no regularly recurring accent.

Syncopation arises when events do not fall exactly on the main beat, provided the beat is still felt. Isorhythms are created when events recur at fixed intervals. Although originally applied to music, the term isorhythm may also be used to describe the recurring patterns found in radio broadcasting, for instance, where news, station logos or specific programs are always repeated at specific times.

In order to include all sound, a third classification must be added which can be called non-metric or free rhythm, where all time intervals are not fractions or multiples of a beat, nor is there any precise regularity of measure or accent. In this situation, each duration simply adds to the others, and so this type of rhythm could be called additive. Wilfrid Mellers, in his book Caliban Reborn (Harper & Row, 1967, p. 3 ff.), proposes the adjectives 'corporeal' and 'spiritual' to describe metric and non-metric rhythms respectively, emphasizing the derivation of the former from gravity-bound, human motor activity, and the latter from religious ecstasy and flights of the spirit.

Additive rhythm is created by randomly occurring events (e.g. the sounds of birds and insects, or the pattern of rain falling). However, the rhythm of many quasi-continuous sounds, such as the wind, may be called additive, since there is a lack of punctuation into separate events. Compare: Stochastic Process and its sound examples.

Punctuation or points of articulation normally occur by means of accents, silence, and changes in content, sound quality or style. Thus it is largely a matter of individual perception and conditioned learning. One person may punctuate a given sequence of events differently from someone else. Robert Ornstein, in his essay On the Experience of Time (Penguin, 1969) goes further to hypothesize an information-processing model of time perception in which the experience of time is conditioned by the way in which information is coded and stored in memory.