The Doppler Effect
Have you ever stood along side a road while a car passed by with its
horn blowing? If so, you probably noticed that the pitch of the sound
was higher when the car was approaching than after it passed. This is
called the Doppler effect.
What you need:
• Tape recorder
• Someone with a car
Try This:
1. Go out in the
country with someone who can drive a car. Take along a battery-operated
tape recorder. If you have a pair of walkie-talkies, take them along
too.
2. Find a long, straight road where there are no
houses or other cars. Find a place where you can safely sit or stand
off to the side of the road with the tape recorder.
3. Have the driver drive past you three times, once
at 20, once at 30, and once at 40 miles per hour. Each time have the
driver blow the horn for a few seconds as the car passes. Your job is
to start and stop the tape recorder at the right times to record the
sounds. You can also record on the tape in your own voice what's
happening. Also make a recording of what the horn sounds like when the
car is not moving. Have the driver drive past without telling you the
speed. Can you estimate the speed from the pitch of the sound?
What’s going on?
The Doppler effect occurs for all kinds of waves. It happens when
either the source of the waves or the receiver of the waves is moving.
The picture shows that the sound waves get compressed as the car moves
toward you, creating a higher pitch sound. As the car moves away, the
sound waves get spread out and you hear a lower sound. The speed of
sound is only 769 mph, so a car going 50 mph would make a big
difference! Could you observe the Doppler effect of water waves if you
were in a moving boat?
The police use radar to measure the speed of cars. Radar works by
the Doppler effect except that it uses radio waves instead of sound
waves. Where do the radio waves come from?
Back to the Just for Kids Page
Wonders of Physics
University of Wisconsin
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Madison, WI 53706
