The Dopper Effect: Sound changes pitch as a moving object passes by. The following movie illustrates this effect as a train passes by two boys. A red arrow points to the boys. An animation showing how the sound waves are compressed at the front of the train and stretch as it passes by will display across the bottom of the movie.

Click the play button to start the movie.
Click the sound button on the play bar to turn up the volume.

Train video excerpt from The Passenger Train (1940) from the Prelinger Archives.