Which of the following best defines the phenomenon of acoustic beats?
a) A periodic variation in sound intensity produced when two sound waves of substantially different frequencies are superposed.
b) A periodic variation in sound intensity produced when two waves of nearly equal but not identical frequencies are superposed.
c) The dramatic increase in loudness that occurs when a sound source or driving force matches the natural frequency of an object (resonance).
d) The pattern of nodes and antinodes that forms when a sound wave interferes with its own reflection, creating a standing wave.