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# Doppler Effect

## Sounds coming from a moving source seem to change in pitch as their distance from the listener changes.

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Doppler Effect

Has this ever happened to you? You hear a siren from a few blocks away. The source is a police car that is racing in your direction. As the car approaches, zooms past you, and then speeds off into the distance, the sound of its siren keeps changing in pitch. First the siren gets higher in pitch, and then it suddenly gets lower. Do you know why this happens? The answer is the Doppler effect.

### What Is the Doppler Effect?

The Doppler effect is a change in the frequency of sound waves that occurs when the source of the sound waves is moving relative to a stationary listener. (It can also occur when the sound source is stationary and the listener is moving.) The Figure below shows how the Doppler effect occurs. The sound waves from the police car siren travel outward in all directions. Because the car is racing forward (to the left), the sound waves get bunched up in front of the car and spread out behind it. Sound waves that are closer together have a higher frequency, and sound waves that are farther apart have a lower frequency. The frequency of sound waves, in turn, determines the pitch of the sound. Sound waves with a higher frequency produce sound with a higher pitch, and sound waves with a lower frequency produce sound with a lower pitch. For an interactive animation of a diagram like the one below, go to the following URL. http://www.colorado.edu/physics/2000/applets/doppler.html

### Experiencing the Doppler Effect

As the car approaches listener A, the sound waves get closer together, increasing their frequency. This listener hears the pitch of the siren get higher. As the car speeds away from listener B, the sound waves get farther apart, decreasing their frequency. This listener hears the pitch of the siren get lower. You can experience the Doppler effect with a moving siren in the following animation: http://www.epicphysics.com/physics-animations/doppler-effect-animation/

Q: What will the siren sound like to listener A after the police car passes him?

A: The siren will suddenly get lower in pitch because the sound waves will be much more spread out and have a lower frequency.

### Summary

• The Doppler effect is a change in the frequency of sound waves that occurs when the source of the sound waves is moving relative to a stationary listener.
• As the source of sound waves approaches a listener, the sound waves get closer together, increasing their frequency and the pitch of the sound. The opposite happens when the source of sound waves moves away from the listener.

### Practice

At the following URL, observe the Doppler effect in the animation and then answer the question. Check your answer by reading the “Discussion” section of the Web page. http://www.ndt-ed.org/EducationResources/HighSchool/Sound/dopplereffect.htm

### Review

1. Define the Doppler effect.
2. Explain why the Doppler effect occurs.
3. Does the siren of a moving police car seem to change pitch to the police officers inside the car? Why or why not?

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