A Visual Model of the Doppler Effect

• What causes the Doppler Effect?
• How does the distance between soundwaves effect pitch?

Vrooooooom! Vrooooooom! Here we are folks in the final lap of the Doppler 500 and look at those cars speed by. Let’s cheer them on by making racecar sounds… Vroom! Vroom! Let’s do it again… Vroom! Hey, did you notice how your racecar voice starts out high and ends up low? Let’s do it again. Vrooom! That’s right, that’s how race cars really sound when they go by. But why? It is because of something called the Doppler Effect. Sound moves from the car to you in waves. Lets make a model of these waves.

• Construction paper
• Scissors
• Tape
• Small toy car

1. Cut the construction paper long-ways into strips about 1” wide.
2. Cut the strips into lengths of decreasing size, each strip being about 2” shorter that the previous. (If you are working with smallish paper, begin by taping strips together so as to get more length. It doesn’t matter exactly how long the strips are as long as they regularly decrease in length).
3. Now tape the ends of each strip together to make circles.
4. Lets pretend these paper circles are sound waves. When the car is still, sound waves move outward from the car. Lay the circles on the floor inside each other and put the racecar right in the middle. This is a model of sound coming from an idling, not moving, racecar.
5. Now, we're goig to race the car. Pretend it’s going fast, but we'll do it in slow motion so we can see what’s happening. Drive the car in one direction, letting it push on the rings in front of it. What’s happening to the rings? (The rings, being pushed by the toy car, should be getting bunched together in front of the car and getting farther apart behind it.) That is exactly what happens to the sound waves as a racecar speeds by you. The waves in front of the car get closer together and the waves behind the car get further apart. Close together waves make a high sound. Far apart waves make a low sound. Thus, the Doppler Effect. Vroom!

Terms/Concepts: doppler effect, sound waves, pitch, high, low, model

References: Galactic Cookie Dough, 50 Hands-On Science Experiments That Explore Astronomy, by B.K. Hixson, pp. 174-177 (Loose in the Lab, Inc., 2003).