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A siren is fitted on as car going toward...

A siren is fitted on as car going towards a vertical wall at a speed of 36 km/h. A person standing on the ground, behind the car, listens to the siren sound coming directly from the source as well as that coming after refelction from the siren to the person and b. coming after reflection. Take the speed of sound to be `340ms^-1`

Text Solution

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Here, speed of car, `upsilon_(s)=36km//h=10m//s`,
speed of sound, `upsilon=340m//s`
(a) In this case, listener is at rest, `upsilon_(L)=0` and source (car) is moving away from listener.
`:.v^(')=(upsilon)/(upsilon+upsilon_(s))v=(340xx500)/(340+10)=485.7Hz`
(b) The frequency received by the wall , when source is moving towards the wall is
`v^('')=(upsilon)/(upsilon-upsilon_(s))xxv=(340xx500)/(340-10)=515.2Hz`
The all reflects this sound without changing the frequency. Therefore, frequency of reflected wave as heard by the listener on the ground is `515.2Hz`.
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