Doppler Effect Classroom Experiments
An ambulance with its sirens blasting sounds quietest as it approaches from a distance and loudest at the point it passes you. It then fades as it drives away. This difference in sound at each location is a summary of the Doppler effect. Teaching this scientific concept to students requires an audible sound device and a little creativity. When students can see and hear the Doppler effect, they will learn the science of sound waves and frequency in no time.
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Basic
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The Doppler effect is a scientific concept proposed by Austrian physicist Christian Doppler in 1842. Doppler discovered a change in wave frequency for the observer as he moved closer or farther away from the sound. This change in wave frequency results in a volume and intensity change in the sound. When the sound is approaching, it is louder than the same sound as it recedes. When the sound is at the instant of passing by the observer, it is identical to its emitted frequency and volume.
Science
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To understand the science, you can visualize the sound waves as actual waves pulsating through the air. The sound waves move through the air until they reach an observer who can hear them. The farther the observer is from the place where the sound is originally emitted, the lower the sound will be from its emitted volume. In an ambulance, the source of the sound is simultaneously moving either toward or away from the observer. When the ambulance is moving closer, the sound waves have to travel less and less of a distance to reach the observer. Visualize this as sound waves bunching together to create a higher frequency and more intense sound. As the ambulance moves away, the sound waves do the opposite and spread out again. The farther the sound source moves away from the observer, the longer the waves take to reach her ears and the quieter the sound. The exact volume of the sound is a result of the motion of the medium, the motion of the observer and the motion of the source.
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Experiment 1
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Create moving sound with a small radio and an apparatus with wheels, such as a computer chair or television cart. These items are often available for free in schools. You can have any sound emit from the radio for this experiment, but a repetitive sound will articulate the effect best. Try to find a recording of a constant beep or an ambulance alarm. Clear the classroom of chairs and desks and set the radio on the apparatus. Split your students into two equal teams and have them form a single file line on each side of the classroom, facing each other. Secure the radio to the apparatus and turn it on. Have a student on one side of the classroom push it over to another student on the opposite side of the room. Students then proceed to push the cart back and forth as the sound is playing. They can push it straight forward or in a diagonal. Ask students to observe the way the sound changes at every position, angle and distance.
Experiment 2
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Tie a musical triangle securely to a long piece of string. Bang the triangle against a hard surface and let students listen to the sound as it travels. Have students switch seats and repeat the process until each student has sat at the front, middle and back of the classroom. Now repeat this process, but twirl the triangle like a lasso around your head after you bang it on a solid surface to produce sound. Let students experience this from all positions in the classroom. Finally, repeat the process with the triangle but have students rotate constantly around the classroom as sound travels. Students will experience the Doppler effect when they and the object emitting the sound are stable, when the object emitting the sound is moving, and when both they and the object emitting the sound are moving.
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References
- Photo Credit Eric Kayne/Getty Images News/Getty Images
