Question
The sirens of two fire engines have a frequency of 650 Hz each. A man hears the sirens from two engines, one moving with speed 10 m/sec and second with speed 54 km/h. What is the difference in the frequency of two sirens heard by the man, when:
(i) both the engines are approaching him,
(ii) both the engines are receding from him?
Take the speed of sound to be 330 m/s.
Solution
The apparent frequency of siren from approaching engine is,
and that from receding engine is,
(i) The apparent frequency of sirens from approaching engines are,
and 
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The difference in the frequencies of wave received from two engines is,
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(ii) The apparent frequency of sirens from receding engine are given by,
and 
The difference in the frequency of two sirens as heard by the man is given by,
