Why does a passing siren change pitch?

1. An ambulance races past Mia on a practice run. As it comes closer its siren sounds high, and as it drives away the sound drops lower. But the siren never changed its note.
2. When the truck is parked, its sound spreads out in evenly spaced rings, like ripples in a pond. Everyone around it hears exactly the same note.
3. Now the truck moves. It sends out each new ring from a spot a bit farther along the road. So the rings crowd together in front of the truck and spread apart behind it.
4. Mia stands in front of the truck. The squeezed waves reach her ears more often each second. More waves each second means a higher pitch.
5. Once the truck has passed, Mia is behind it. The stretched waves arrive less often, so she hears a lower pitch. That sudden drop is the neee-yowww.
6. People use this effect to measure speed. A radar sign bounces waves off a car. The faster the car comes, the more squeezed the waves bounce back.

The truck drives along, sending out sound rings. Tap each part of the picture to see what happens there.

• Siren. The siren sends out sound waves at a steady rate. Its own note never changes, however fast the truck goes.
• Squeezed waves. The truck chases its own waves. Each new ring starts closer to the last one, so the waves in front are squeezed together.
• Stretched waves. The truck drives away from the waves it sends backward. Each new ring starts farther from the last one, so the waves behind are stretched out.
• Listener in front. Squeezed waves reach this listener more often each second, so she hears a higher note than the siren is really making.
• Listener behind. Stretched waves reach this listener less often each second, so he hears a lower note than the siren is really making.
• Driver. The driver moves along with the siren, so the distance between them never changes. She hears the true note the whole time.

Drag the truck past Mia, or press Drive past. Compare the wave spacing in front of the truck and behind it, and watch the pitch Mia hears.

Experiment: The truck drives past Mia at a steady speed. What does she hear?

1. Park the truck and look at the rings. Are they evenly spaced?
2. Press Drive past and watch the rings in front of the truck.
3. Keep watching after the truck passes Mia. What happens to the pitch?

What we found: The waves in front of the truck are squeezed together, so Mia hears a higher note while it comes closer. The waves behind it are stretched apart, so she hears a lower note once it has passed. The siren itself never changes.

Turn on JavaScript to try it yourself.

A truck is hidden behind the hedge. Use the spacing of the waves that reach Mia to decide what it is doing. Four trucks to solve.

Stand by a road when an ambulance goes by and you will hear it: a high neeee as it comes closer, and a lower yowww as it drives away. The siren is not changing its note. The driver hears the same steady sound the whole time. The change happens because the truck is moving.

Sound spreads out from a siren in rings, like ripples from a pebble dropped in a pond. If the truck is parked, the rings are evenly spaced. But a moving truck sends out each new ring from a spot a little farther along the road. In front of the truck, each ring starts closer to the one before, so the rings are squeezed together. Behind it they are stretched apart.

Your ear decides how high a sound is by how many waves reach it each second. Squeezed waves arrive more often, so you hear a higher pitch. Stretched waves arrive less often, so you hear a lower pitch. As the truck passes you, you switch from squeezed waves to stretched ones, and the pitch drops.

A duck on a pond shows the same pattern in its ripples, but sound waves spread in every direction, up and down as well. For a real ambulance the change is small, about three keys on a piano. Our model truck is much faster than a real one, so that you can see the squeezing clearly.

Key words

Pitch

How high or low a sound is.

Frequency

The number of waves each second, measured in hertz (Hz).

Wavelength

The distance from one wave to the next.

Doppler effect

The change in pitch you hear when a sound source moves toward you or away from you.

Wavefront

One ring of a spreading wave, where the push of the sound is at its strongest.

Check your thinking

1. An ambulance is driving toward you. Compared with its real note, what do you hear?

   Show the answer

   A higher note. The waves in front of a moving source are squeezed together, so more reach you each second.

2. Where are a moving truck's sound waves closest together?

   Show the answer

   In front of it. The truck sends out each new wave from farther forward, so the waves ahead crowd together.

3. What does the ambulance driver hear while driving?

   Show the answer

   The real note, all the time. The driver moves along with the siren, so the distance between them never changes.

4. The truck drives faster. What happens to the change in pitch as it passes?

   Show the answer

   The change gets bigger. A faster truck squeezes the waves in front more and stretches the ones behind more.

5. Light from a distant galaxy is shifted toward lower frequencies (toward red). What does that tell astronomers?

   Show the answer

   The galaxy is moving away from us. Waves from a source that is moving away are stretched, so their frequency is lower. For light that is a shift toward red.