Question

You are at a street corner and hear an ambulance siren. Without looking how can you tell when the ambulance passes by?

Answer 1

It is a case in which, source of sound recedes away from a stationary observer. By Doppler's effect we know that when source recedes away from a stationary observer , frequency (pitch) of sound decreases . Therefore pitch of the siren will drop and we come to know that ambulance is going away .