Doppler Effect: sound waves emitted from a moving vehicle.
This animation shows why the engine sound of a passing car is high when it approaches you and drops as it goes past. The sound waves are represented by pale yellow expanding circular wave fronts at ground level (the wave fronts are really spherical and these rings represent their intersection with the ground). As the car moves forward, sound waves are emitted at different points along the road. Each wave front continues to expand from its point of origin. Consequently, the waves are closer together at the front of the car and more spread out behind. The crowded sound waves ahead of the car give the engine sound a higher frequency, while the more widely spaced ones behind yield a lower pitch. Speed and scales are distorted for clarity. In reality, sound will be much much faster than the car. Only a jet travelling at the speed of sound will catch up with the advancing wave fronts.