Watch how wavefronts bunch up ahead of a moving source and spread out behind it.
Why does a siren sound higher as it comes toward you, then lower as it drives away?
When a sound source is moving toward you, it's getting closer with every new sound wave it sends out. This squishes the waves together so they hit your ear more often, and more waves per second means a higher pitch.
Once the source passes you and starts moving away, the opposite happens. Now each wave has to travel farther to reach you, so the waves spread out more. Fewer waves hit your ear per second, and you hear a lower pitch.
This is the Doppler effect. The faster the source moves relative to the speed of sound, the larger the change in pitch. If the source moves at Mach 1 (meaning it is going exactly the speed of sound), all the waves stack up on top of each other in front of it. Go even faster than that and a shock-wave cone forms behind the source creating a sonic boom. This is what you hear when a jet breaks the sound barrier.
leoniscool.com