Question

A source of sound of frequency $256H_z$ is moving rapidly towards a wall with a velocity of $5m{s}^{-1}$ how many beats per second will be heard if sound travels at a speed of $330m{s}^{-1}$?

• A. 5 Hz
• B. 6 Hz
• C. 7 Hz
• D. 8 Hz

Answer 1

The correct option is D 8 Hz

When the source moves towards the wall


Apparent frequency $=f=\frac{fv}{V-V_S}$
$f_1=\frac{256\times 330}{330-5}=260Hz$
When the reflected sound moves away from the source
$f_2=\frac{fV}{V+V_s}$
$=\frac{256\times 330}{330+5}=252Hz$
Number of beats heard = (260 – 252) = 8Hz