A whistle of frequency 540 Hz 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 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
`f_(max)=((V)/(V-V_(S)))f=((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^(@) or V_(s) cos theta=V_(s) cos90^(@)=0`
So, there will be no Doppler effect.