How does the Doppler effect change the pitch of a passing car horn?

The Doppler effect describes the change in the perceived frequency of sound waves caused by the relative motion between a source and an observer.12

When a car horn approaches you, the sound waves are compressed, causing them to reach your ears at a higher frequency, which you perceive as a higher pitch. As the car passes and begins to move away, the waves are stretched out, reaching you at a lower frequency and resulting in a lower perceived pitch. This phenomenon occurs because the distance between the source and the observer is either decreasing or increasing, directly influencing the frequency of the waves received.12