The Doppler applet is a part of the Physlets collection of Prof. W. Christian at Davidson University.

Vary the velocity using the slider and notice how the waves are compressed ahead of the source and stretched out behind the source. If the velocity becomes too large, the source overtakes its own waves and a v-shaped wake is produced that is called a shock wave; for sound, it is the "sonic boom" produced by a plane breaking the sound barrier.

The formula in the left frame is valid only for v < < c. The Doppler applet allows you to use that formula (Classical) or one that is also correct when v approaches c (Relativistic).

Right click the applet window with your mouse to access a help file with more detailed instructions.