system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the sound of an engine change when it drives past you?

Cars, planes, jets anything with a motor.
gpt
The Doppler Effect is super easy to understand with a good animated diagram, that you can find easily with Google. But I’ll try to explain it in text.

Imagine your annoying friend is pelting you with balls. He throws the balls at you at a constant rate, for example once every second.

If he’s running toward you while throwing the balls, the balls hit you more frequently. That’s because each ball has less distance to travel. As your friend gets closer to you, the balls can reach you more quickly.

On the other hand, if he’s running away from you, each ball has more distance to cover than the last, so you don’t get hit as frequently.

Sound waves are like the balls, and the pitch of a sound depends on its frequency. When the source of sound is moving toward you, the sound waves hit you more frequently, so you hear a higher pitched sound, and the opposite when it’s moving away.