Q. A source of sound of frequency $256\ Hz$ is moving rapidly towards a wall with a velocity of $5m/s.$ The speed of sound is $330\ m/s.$ If the observer is between the wall and the source, then beats per second heard will be
A) $7.8\ Hz$
B) $7.7\ Hz$
C) $3.9\ Hz$
D) Zero
Explanation
(d) The observer will hear two sounds, one directly from source and other from reflected image of sound
[image:15_image62.png]
Hence number of beats heard per second
$= \left( \frac{v}{v - v_{S}} \right)n - \left( \frac{v}{v - v_{S}} \right)n = 0$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.