Music is a supporting component of many kinds of entertainment and most kinds of performance. For example, it is used to enhance storytelling, it is indispensable in dance (1) and opera, and is usually incorporated into dramatic film or theatre productions. Music is also a universal and popular type of entertainment on its own, constituting an entire performance such as when concerts are given (2, 4, 5, 6, 7, 8, 9 ). Depending on the rhythm, instrument, performance and style, music is divided into many different genres, such as classical, jazz or folk (3, 5, 8), and rock (6, 9). Since the 20th century, performed music, once available only to those who could pay for the performers, has been available cheaply to individuals by the entertainment industry which broadcasts it or pre-records it for sale. The wide variety of musical performances, whether or not they are artificially amplified (4, 6, 7, 9, 10), all provide entertainment irrespective of whether the performance is from soloists (4, 6), choral (2) or orchestral groups (5, 8), or ensemble (3). Live performances use specialised venues, which might be small or large; indoors or outdoors; free or expensive. The audiences have different expectations of the performers as well as of their own role in the performance. For example, some audiences expect to listen silently and are entertained by the excellence of the music, its rendition or its interpretation (5, 8). Other audiences of live performances are entertained by the ambience and the chance to participate (7, 9). Even more listeners are entertained by packaged music and list
n privately (10). The instruments used in musical entertainment are either solely the human voice (2, 6) or solely instrumental (1, 3) or some combination of the two (4, 5, 7, 8). Whether the performance is given by vocalists or instrumentalists, the performers may be soloists or part of a small or large group, in turn entertaining an audience that might be individual (10), passing by (3), small (1, 2) or large (6, 7, 8, 9). Singing is generally accompanied by instruments although some forms, notably a cappella and overtone singing, are unaccompanied. Rhythm (from Greek —rhythmos, "any regular recurring motion, symmetry") may be generally defined as a "movement marked by the regulated succession of strong and weak elements, or of opposite or different conditions." This general meaning of regular recurrence or pattern in time may be applied to a wide variety of cyclical natural phenomena having a periodicity or frequency of anything from microseconds to millions of years. In the performance arts rhythm is the timing of events on a human scale; of musical sounds and silences, of the steps of a dance, or the meter of spoken language and poetry. Rhythm may also refer to visual presentation, as "timed movement through space." and a common language of pattern unites rhythm with geometry. In recent years, rhythm and meter have become an important area of research among music scholars. Recent work in these areas includes books by Maury Yeston, Fred Lerdahl and Ray Jackendoff, Jonathan Kramer, Christopher Hasty, Godfried Toussaint, William Rothstein, and Joel Lester.