Question

A source of sound of frequency $\text{256 Hz}$ 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. $\text{7.8 Hz}$
• B. $\text{7.7 Hz}$
• C. $\text{3.9 Hz}$
• D. Zero

Answer 1

The correct option is A $\text{7.8 Hz}$

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

Hence number of beats heard per seconds

$\left(\frac{V}{V-V_s}\right)n-\left(\frac{V}{V+V_s}\right)n$

$\frac{2nV{V}_s}{V^2-V_s^2}=\frac{2\times 256\times 330\times 5}{335\times \times 325}=7.8KHz$