Animation of Doppler effect: moving source and stationary listener.
For a moving source and stationary listener, we see that the sound wave crests are compressed in the direction of motion and rarefied in the direction opposite the motion of the sound source.
In this animation of doppler effect moving source case, the source emits a 400Hz tone but the listener hears a 420Hz tone at first because the source is moving toward the listener. The motion of the source toward the listener compresses the wave crests to a shorter wavelength, thus a higher frequency sound.
The tone ramps down to 380Hz as the source passes the listener and heads away. With the source moving away from the listener, the wave crests are stretched out, giving a longer wavelength and therefore a lower frequency sound.
