Question

A source of sound vibrating with a frequency 510Hz is in between a stationary observer and a foot wall. If the source is moving towards the wall with a velocity 3 m/s, the number of beats heard by the observer is : (velocity of sound in air=340m/s)

• A. 3 beats/s
• B. 6 beats/s
• C. 9 beats/s
• D. 4.5 beats/s

Answer 1

The correct option is C 9 beats/s

When two sound waves of different frequency approach your ear, the alternating constructive and destructive interference causes the sound to be alternatively soft and loud - a phenomenon which is called "beating" or producing beats.
Here the source frequency is $510Hz$. When the source will start moving towards the wall with a velocity $V_s$ then the frequency of the source will be change to the stationary observer. The frequency of the beat will be:
Beat Frequency $=\frac{2\times \nu \times V_s}{V}$
Putting all the values in above equation will get,
Beat frequency = $\frac{2\times 510\times 3}{340}=9beats/s$