question_answer

A musician using an open flute of length 50 cm produces second harmonic sound waves. A person runs towards the musician from another end of a hall at n speed of 10 km/h. If the wave speed is 330 m/s, the frequency heard by the running person shall be close to:                                                                    [JEE Main 09-Jan-2019 Evening]

A)  753 Hz                        

B)  666 Hz

C)  500 Hz                        

D)  333 Hz

Correct Answer: B

Solution :

[b] Frequency of sound source \[=\,\,\frac{2\,V}{2\,L}\]

\[=\,\,\frac{330}{50\,\times \,{{10}^{-2}}}\,=\,660\,Hz\]

Speed of observer \[=\,\,\frac{10\,\,\times \,\,5}{18}\,m/s\]

Apparent frequency

\[=\,\,2\left[ 330+\frac{50}{18} \right)\,\,\,=\,\,666\,Hz\]