Written by W. Christian at Davidson College.

Additional comments by J. Varriano:
Classical
The applet shows a moving object emitting sound waves.
The number in the box on the right shows the Mach number which is the ratio of the speed of the object to the speed of sound.

Relativistic
The applet shows a moving object emitting light waves.
The number in the box on the right shows the ratio of the speed of the object to the speed of light.