Waves

An observer is riding on a bicycle and moving towards a hill at \(18 \,\mathrm{kmh}^{-1}\). 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 \(640 \mathrm{~Hz}\) and velocity of the sound in air is \(320 \mathrm{~m} / \mathrm{s}\), the beat frequency between the two sounds heard by observer will be _____________ \(\mathrm{Hz}\).