An observer is riding on a bicycle and moving towards a hill at 18 kmh^-1. He hears a sound from a source at some distance behind him directly as…

An observer is riding on a bicycle and moving towards a hill at $\displaystyle 18 \mathrm{kmh}^{-1}$. He hears a sound from a source at some distance behind him directly as well as after its reflection from the hill. If the original frequency of the sound as emitted by source is $\displaystyle 640$ Hz and velocity of the sound in air is $\displaystyle 320$ m/s, the beat frequency between the two sounds heard by observer will be \_\_\_\_ Hz.