A whistle of frequency `540 H_(Z)` rotates in a circle of radius `2 m`at a linear speed of `30 m//s` . What is the lowest and highest frequency heard by an observer a long distance away at rest with respect to the centre of circle ? Take speed of sound of sound in air as `330 m//s`. Can the apparent frequency be ever equal to actual ?

Explanation

Apparent frequency will be minimum when the source is at N and moving away from the observer.

Apparent frequency will be maximum when source is at L and approaching the observer.
`n_(max).=(v)/(v-v_(s))n=((330)/(330-30))(540)=594Hz`
Further when source is at M and K, angle between velocity of source and line joining source and observer is `90^(@)` (i.e., line of sight is perpendicular to `v_(s)`) or `v_(s)costheta=v_(s)cos90^(@)=0`
So, there will be no Doppler effect