http://www.ck12.org Chapter 5. Energy
5.36 Doppler Effect
- Describe the Doppler effect.
- Explain why the Doppler effect occurs.
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?
TheDoppler effectis 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.) TheFigure5.72 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