A disc of radius .R is rotating uniformly with angular frequency o A source of sound is fixed to the rim of the disc. The ratio of maximum and minimum frequencies heard by stationary observer, far away from the disc and in the plane of the disc is (v = speed of round) 

Two positions of the source arc shown in fig
 

The frequency heard by stationary observer will be maximum when the source is approaching him. Therefore,
$$\begin{gathered} {\mathrm{n}}_{max}=\left(\frac{\mathrm{v}}{\mathrm{v}-{\mathrm{v}}_{\mathrm{s}}}\right)\mathrm{n}=\left(\frac{\mathrm{v}}{\mathrm{v}-\mathrm{R\omega }}\right)\mathrm{n} \\ \mathrm{The} \mathrm{frequeacy} \mathrm{heard} \mathrm{by} \mathrm{stationary} \mathrm{observer} \mathrm{will} \mathrm{be} \\ \mathrm{minimum} \mathrm{when} \mathrm{the} \mathrm{source} \mathrm{is} \mathrm{receding} \mathrm{him}, \mathrm{Therefore}, \\ {\mathrm{n}}_{min}=\left(\frac{\mathrm{v}}{\mathrm{v}+{\mathrm{v}}_{\mathrm{s}}}\right)\mathrm{n}=\left(\frac{\mathrm{v}}{\mathrm{v}+\mathrm{R\omega }}\right)\mathrm{n} \\ \text{ So, }\frac{{\mathrm{n}}_{max}}{{\mathrm{n}}_{min}}=\left(\frac{\mathrm{v}+\mathrm{R\omega }}{\mathrm{v}-\mathrm{R\omega }}\right) \end{gathered}$$