Ubi Caritas et amor Deus tibi est.
If you are standing by the side of a road and an ambulance passes by, or even a car with its horn sounding, you will hear the doppler effect.
Click the speaker to hear the sound of the doppler effect.
The sound you have heard is an example of the Doppler effect.
What you hear is a change in the frequency of the sound as the car approaches and passes.
The small cartoon below will help visualise how the sound waves from theb car`s horn appear in front and behind the car`s direction of motion.