This Demonstration is a visualization of the Doppler effect. As time increases, the blue object moves along the road and its velocity relative to the green observer ) changes. As a result, the sound that the blue object emits is heard by the observer at a different frequency than that at which it is originally emitted. This phenomenon is called the Doppler effect.
This Demonstration plays tones at the frequency that the observer would hear as time passes and the object moves along the road. It is recommended that the user animate the time slider in order to allow the scenario to play at a constant speed.