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Beat (acoustics)

In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, perceived as a periodic variation in volume whose rate is the difference of the two frequencies.

With tuning instruments that can produce sustained tones, beats can be readily recognized. Tuning two tones to a unison will present a peculiar effect: when the two tones are close in pitch but not identical, the difference in frequency generates the beating. The volume varies like in a tremolo as the sounds alternately interfere constructively and destructively. As the two tones gradually approach unison, the beating slows down and may become so slow as to be imperceptible. As the two tones get further apart, their beat frequency starts to approach the range of human pitch perception,[1] the beating starts to sound like a note, and a combination tone is produced.

Uses[edit]

Musicians commonly use interference beats objectively to check tuning at the unison, perfect fifth, or other simple harmonic intervals.[8] Piano and organ tuners use a method involving counting beats, aiming at a particular number for a specific interval.


The composer Alvin Lucier has written many pieces that feature interference beats as their main focus. Italian composer Giacinto Scelsi, whose style is grounded on microtonal oscillations of unisons, extensively explored the textural effects of interference beats, particularly in his late works such as the violin solos Xnoybis (1964) and L'âme ailée / L'âme ouverte (1973), which feature them prominently (Scelsi treated and notated each string of the instrument as a separate part, so that his violin solos are effectively quartets of one-strings, where different strings of the violin may be simultaneously playing the same note with microtonal shifts, so that the interference patterns are generated). Composer Phill Niblock's music is entirely based on beating caused by microtonal differences.[9] Computer engineer Toso Pankovski invented a method based on auditory interference beating to screen participants in online auditory studies for headphones and dichotic context (whether the stereo channels are mixed or completely separated).[10]

Autonomous sensory meridian response (ASMR)

Consonance and dissonance

Gamelan tuning

Heterodyne

a form of spatial interference that generates new frequencies.

Moiré pattern

Music and sleep

Voix céleste

Thaut, Michael H. (2005). Rhythm, music, and the brain : scientific foundations and clinical applications (1st in paperback ed.). New York: Routledge.  978-0415973700.

ISBN

Berger, Jonathan; Turow, Gabe, eds. (2011). Music, science, and the rhythmic brain : cultural and clinical implications. Routledge.  978-0415890595.

ISBN

Learning materials related to Beat (acoustics) at Wikiversity

MIT

Javascript applet

D.A. Russell, Pennsylvania State University

Acoustics and Vibration Animations

A Java applet showing the formation of beats due to the interference of two waves of slightly different frequencies

Lissajous Curves: Interactive simulation of graphical representations of musical intervals, beats, interference, vibrating strings

The Feynman Lectures on Physics Vol. I Ch. 48: Beats