How Does The Doppler Effect Change Sound Pitch?

Imagine a dog chasing its tail. When it runs forward, the hair on its back gets squished together tight. When it stops, that fur puffs out again. Sound works much the same way! Doppler effect is what happens when a sound maker moves toward or away from you.

Sound travels in waves, like ripples in a pond. If a car honks its horn while standing still, the ripples spread out evenly. But if that car drives toward you, it catches up to the waves it just made. This squishes the waves closer together. Closer waves mean your ears hear them faster, which sounds like a higher pitch.

When the car passes and drives away, it is running from its own ripples. The waves get stretched out longer. Longer waves arrive slower, so you hear a lower pitch. This is why an ambulance siren sounds high-pitched as it approaches, then drops to a low groan as it zooms past.