Whistle of frequency 540 Hz rotates in a circle of radius 2m at 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 at sound in air as 330 m/s.

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

$\begin{array}{l}f\min =\left[\frac{V}{V+V_s}\right]f\\ =\left[\frac{330}{330+30}\right]540\\ =495Hz\end{array}$
 
Frequency will be maximum when source is at L and approaching the observer
 $\begin{array}{l}f\min =\left[\frac{V}{V-V_S}\right]f=\left[\frac{330}{330-30}\right]540\\ =594Hz\end{array}$