A person driving a car at a constant speed of 15 m/s is approaching a vertical wall. [2023]

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Solution

Q.
A person driving a car at a constant speed of 15 m/s is approaching a vertical wall. The person notices a change of 40 Hz in the frequency of his car’s horn upon reflection from the wall. The frequency of horn is __________ Hz. (Given: Speed of sound is 330 m/s) [2023]

Ans.
(420)

$Frequency of reflected sound = (\frac{v + v_{c}}{v - v_{c}}) f_{0}$

$f = (\frac{330 + 15}{330 - 15}) \times f_{0} = \frac{345}{315} f_{0}$

$\frac{345}{315} f_{0} - f_{0} = 40 \Rightarrow \frac{30}{315} f_{0} = 40$

$f_{0} = \frac{4 \times 315}{3} = 420 Hz$

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