Doppler & Reflection of Sound

Doppler effect
The change in frequency experienced by the receiver either because of the relative motion of the source or the receiver or both is Doppler Effect.
There is an increase in frequency of the whistling from fast train when it comes from a distance to the station and a decrease in frequency when the train passes us without stopping. This is due to the Doppler Effect. This phenomenon was first explained by an Austrian scientist Christian Andreas Doppler.