Racing Against Sound: Doppler Effect
Weather forecasters use the Doppler effect to detect wind shear. Astronomers use it to determine that distant galaxies are moving away from each other in an apparent expansion of the universe. This experiment demonstrates how the frequency of a sound moving toward or away from a listener is affected by the Doppler effect.
What You Need
- 1 meter length of string
- 1 electric buzzer or other source of a sustained note (the buzzer must have a point of attachment for a piece of string and must not require someone to continuously activate a switch to make it sound)
- 2 people
- optional: microphone, oscilloscope, or sound card oscilloscope
- Securely attach the buzzer to the string.
- Turn on the buzzer.
- One person spins the buzzer in a circle, moving toward and away from the second person, as shown in Figure 74-1.
- The observer should listen to the sound the buzzer makes as it comes toward and away from where they are located.
The pitch of the sound increases and decreases at a rate established by the period of the rotating buzzer. The volume of the buzzer sound may also increase and decrease, but that is not the Doppler effect. The faster the buzzer spins, the greater the difference in pitch.
The pitch is higher as the sound moves toward you and lower as it moves away from you.
Why It Works
When sound is coming toward you, the peaks and troughs of the waves are closer together, as indicated in Figure 74-2. This results in a higher frequency of the sound wave. From the perspective of the listener, the sound waves seem to come more frequently and are perceived to have a higher pitch.
Other Things to Try
Attach the microphone to either a physical oscilloscope or a sound card oscilloscope.
Display the sound and compare the frequency produced by the buzzer coming and going.
Attach a buzzer to any object that can move in a more or less horizontal direction (such as an air track glider, a frictionless cart, or even an old skateboard). As the object moves, listen to the sound. If you can, also try displaying the waveform on an oscilloscope.
The Doppler effect occurs when the source of the sound is either moving toward you or away from you. When the source of sound is moving toward you, the pitch or frequency of the sound is higher; when the source of sound is moving away from you, the pitch is lower.
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