The Doppler effect
Race a siren across the screen and see its sound waves bunch up ahead and stretch out behind, then hear the pitch drop.
Standards: HS-PS4-1
1Wonder
An ambulance races past: neee-yooowww. The siren sounds high as it comes toward you and suddenly drops as it passes. But the driver hears the same steady note the whole time. Who is right, and what happens to the sound on its way to you?
2Explore
This activity needs a bigger screen
Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.
- Set the speed to 0. Are the circles closer together on one side?
- Speed the siren up. Where do the circles bunch up? Where do they spread out?
- Press Hear it pass at a few speeds. How does the jump in pitch change?
- Push the speed past 340 m/s, the speed of sound. What shape do the circles make?
3Explain
Sound spreads in circles
A siren sends out waves of squeezed air, one after another, that spread out in circles at the speed of sound (about 340 m/s). Standing still, the circles are evenly spaced, so everyone hears the same pitch.
When a source of waves moves, the waves bunch up ahead and spread out behind. Ahead you hear a higher pitch; behind, a lower one. Faster than the waves themselves, they pile into a cone: a sonic boom.
4Practice
As an ambulance comes toward you, its siren sounds…
5Recap
- A moving source sends each wave from a new spot.
- Waves bunch up ahead (higher pitch) and spread out behind (lower pitch).
- f′ = f × c ÷ (c − v) toward you, and f × c ÷ (c + v) away from you.
- It works for light too: galaxies moving away look redder.