Question

A source of sound of frequency $256Hz$ is moving rapidly towards a wall with a velocity of $5m/s$. The speed of sound is $330m/s$. If the observer is between the wall and the source, then beats per second heard will be

• A. 7.8 Hz
• B. 6.7 Hz
• C. 3.9 Hz
• D. Zero

Answer 1

Answer: (A)

The observer will hear two sounds, one directly from source and other from reflected image of sound.

Hence number of beats heard per second
$=\left(\frac{v}{v-v_S}\right)n-\left(\frac{v}{v+v_S}\right)n$
$=\frac{2nv{v}_S}{v^2-v_S^2}$
$=\frac{2\times 256\times 330\times 5}{335\times 325}=7.8Hz$