Why does the Doppler effect change the pitch of sound?

Answer: Relative motion changes the frequency of sound waves reaching your ear

Wave reflection changes the speed of sound waves reaching your ear — Wrong. Reflection can create echoes, but the Doppler shift does not require sound to bounce off anything; it comes from motion changing the spacing of waves between source and listener. This misconception gets the everyday echo idea almost right—reflected sound can seem different—but the speed of sound in air is not what changes here. What changes is how frequently the wave crests reach your ear.

Source motion changes the frequency that the source itself emits — Wrong. A moving source does not have to emit a different frequency; its motion changes how its waves are spaced in the air, not the frequency produced at the source. This misconception gets the direction right—approach raises the perceived pitch—but places the change inside the source rather than in how the waves reach the listener. The source can keep emitting the same frequency while a listener hears a different pitch.

Relative motion changes the frequency of sound waves reaching your ear ✓ — Correct! When a source moves toward you, each successive sound wave starts a little closer, so wave crests arrive more often and you hear a higher pitch. When it moves away, the crests arrive less often and the pitch drops. The source still emits the same frequency; relative motion changes the spacing and arrival rate of waves at your ear, which is why a passing siren seems to change pitch.