- Why does the sound of a car going by change from high-pitched to low-pitched?
Have you ever noticed that the sound a car makes is different when it’s coming toward you than when it’s going away from you? Close your eyes and picture it and try to hear the sound in your head—or better yet, go outside where you can (safely!) see and hear cars going by and listen. This project creates a visual model of what’s happening to the sound waves to make them sound different when the car’s approaching than when it’s leaving.
- Two pieces of construction paper in different colors
- Small toy car
- Some blank paper and a pencil, or a camera
- Cut one piece of construction paper the long way into five inch-wide strips. They should all be 11” long. Leave the first strip that long; cut an inch off of the second strip so that it’s 10” long; cut two inches off of the second strip so that it’s 9” long; and so on so that each strip of paper is an inch shorter than the last one.
- Tape the ends of the five strips of construction paper together so they form loops.
- Put the car down on the centre of the second piece of construction paper. Arrange the loops around it so that the car is at the centre and the loops are all centered on the car—none of the loops should be touching the car or each other, and they should all be about the same distance apart from each other. The loops are your sound waves; they show how the sound of the car’s engine travels outward when the car is standing still.
- Draw a picture or take a photo of the car and the loops the way they look now. (Optional: glue everything in place exactly the way it is now, and get a second car, make a second set of loops, and use another sheet of construction paper for the next part!)
- Now gently roll the little car forward until it pushes all the loops in front of it together so that they all touch. This shows what happens to the sound waves when the car is moving: the ones at the front get squished together, or compressed, so that they sound higher-pitched, while the sound waves at the back get spread out or stretched so that the sound they make is lower-pitched. That’s the Doppler effect!
- Draw a picture or take a photo of the car and the loops in this position. (Or if you’re doing the gluing-in-place option, glue all of this in place.)
- Now look at your two models, of the car standing still and the car in motion. You can see how the sound waves are affected by the movement of the car. Can you think of anything else that travels in waves that might be subject to the Doppler effect?
Terms/Concepts: Sound waves, compression, Doppler effect
References: What’s Up? 45 Hands-On Science Experiments That Explore Weather, by B. K. Hixson, pp. 172-173 (Loose in the Lab Science Series, 2003).