- When two sound waves of nearly same frequencies, travelling in a medium, superimpose on each other, the intensity of the resultant sound at a particular position rises and falls regularly with time. - This phenomenon of regular variation in the intensity of sound with time at a particular position is called beats. - Beat frequency: The number of beats produced per second is called a beat frequency. EXPLANATION: - From above it is clear that, the phenomenon of wavering of sound intensity when two waves of nearly the same frequencies and amplitudes travelling in the medium, are superimposed on each other is called beats. Therefore option 4 is correct.