Question

Two sirens separate by 1 km distance are producing a sound of frequency 330 Hz. Velocity of sound is 330 m/s. A boy is moving with speed 2 m/s between the sirens from one to another. Number of beats heard by boy is

• A. 0
• B. 3
• C. 2
• D. 4

Answer 1

The correct option is D 4

${f_1}^{\prime }=\left(\frac{330+2}{330}\right)330=332Hz{f_2}^{\prime }=\left(\frac{330-2}{330}\right)330=328Hz\therefore beat=\left|{f_1}^{\prime }-{f_2}^{\prime }\right|=4$

Hence,

option $\left(D\right)$ is correct answer.