Beat Frequency
Beats are produced by the superposition of two waves of slightly different frequencies but identical amplitudes.The waves alternate in time between constructive interference and destructive interference, giving the resulting wave a time-varying amplitude.
Source
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"OpenStax College, Superposition and Interference. February 9, 2013."
http://cnx.org/content/m42249/latest/Figure_17_10_08a.jpg
OpenStax CNX
CC BY 3.0.
