Question

Which of the following is true regarding beats?

• A. Frequency different, amplitude same
• B. Frequency same, amplitude same
• C. Frequency same, amplitude different
• D. None of the above

Answer 1

Answer: (A)

Frequency different, amplitude same

Answer 2

Number of beats is equal to difference in frequency. When two sound waves of slightly different frequencies, travelling in a medium along the same direction, superimpose on each other, the intensity of the resultant sound at a particular position rises and falls alternately with time.
This phenomenon of alternate variation in the intensity of sound with time at a particular position, when two sound waves of nearly equal frequencies and same amplitude superimpose on each other is called beats.
For the formation of distinct beats, frequencies of two sources of sound should be nearly equal i.e., difference in frequencies of two sources must be small, say less than $10$.
In order to hear distinct beats, time interval between two successive beats must be greater than $1/10$.
Therefore, frequency of beats must be less than $10$, i.e., number of beats/ second, which is equal to difference in frequencies of two sources must, be less than $10$.
Hence, the two sources should be of nearly equal (but not same) frequencies.