The sirens of two fire engines have a frequency of 600 Hz each . A man hears the sirens from two engines, one approaching him with a velocity of `36km//h` and the other going away from him at a speed of `54km//h`. What is the difference in frequencies of sound heard ? Take velocity of sound in air `=340m//s`.

Explanation

Here, fire engines are sources of sound and man is listener, `v=600Hz`
`upsilon_(L)=0, upsilon_(s_(1))=36km//h=10m//s,`
`upsilon_(s_(2))=54km//h=15m//s`
`v^(')=(upsilon.v)/(upsilon-upsilon_(s_(1)))=(340xx600)/(340-10)=618.2Hz`
`v^('')=(upsilon.v)/(upsilon-upsilon_(s_(2)))=(340xx600)/(340+15)=574.6Hz`
Difference in frequencies `=v^(')-v^('')`
`618.2-574.6=43.6Hz`