A man is standing on a railway platform listening to the whistle of an engine that passes the man at constant speed without stopping. If the engine passes the man at time t0. How does the frequency f of the whistle as heard by the man changes with time

When the train is approaching the stationary observer, frequency heard by the observer n' = v-v0vn when the train is moving away from the observer then frequency heard by the observer n'' = v-v0vn  it is clear that n' and n" are constant and independent of time. Also and n' > n".