Question

The sirens of two fire engines have a frequency of 600 Hz each. A man hears the sirens from the two engines, one approaching him with a speed of 30 Kmph and the car going away from him at a speed of 54 Kmph. What is the difference in frequency of two sirens heard by the man?

Answer 1

Velocity of sound be 340 m/s 

Frequency n = 600 Hz Velocity of sound 

V = 340 m/s 

Velocity of source 

V_S = 36 Kmph 

\(\frac{36 \times 5}{18}\)= 10 m/s 

Velocity listener 

V_S = 0

For the engine approaching man

= 618.2 Hz

For the engine going away from the man 

V_s = -54 Kmph 

= -54 ×\(\frac{5}{18}\) = -15 m/s 

Velocity of listener V_L = 0 

Apparent frequency

Difference in frequencies = n’ – n” = 618.2 – 574.6 

= 43.6 Hz