Why does Doppler effect change sound pitch?

Answer: Motion changes wave frequency

Motion changes wave frequency ✓ — Correct! When a sound source moves toward you, sound waves get compressed—you encounter more wave peaks per second, increasing frequency (pitch). When it moves away, waves are stretched—fewer peaks per second, decreasing frequency. The sound itself doesn't change, but relative motion changes how many sound waves reach your ear per second, so changing perceived pitch. Classic example: passing ambulance siren.

Speed creates new sound — Wrong. Motion doesn't create new sound—the source emits the same sound frequency regardless of movement. What changes is how many sound waves per second reach your ear due to relative motion. If a siren approaches at 440 Hz, it still emits 440 Hz, but you receive more waves per second (higher pitch) because you're moving toward incoming waves.

Sound waves bounce back faster — Wrong. The Doppler effect isn't about sound bouncing back. It's about how motion compresses or stretches sound waves as they travel from source to listener. An approaching siren packs more waves into each second, so you hear a higher pitch—no reflection involved.