A whistle of frequency $500\mathrm{Hz}$ tied to the end of a string of length $1.2 \mathrm{m}$ revolves at $400 \mathrm{rev} {\min }^{-1}$. A listener standing some distance away in the plane of rotation of whistle hears frequencies in the range in Hertz. (Speed of sound $=340 \mathrm{m} {\mathrm{s}}^{-1}$)