The Doppler Effect


Everyone hears the weatherman talk about the Doppler radar, but just what is this? In this experiment, we will discover the principles associated with the Doppler radar, and examples of the Doppler Effect that we may hear everyday.

List of Materials:

  • string
  • buzzer or other noisemaker
  • small bag or sock


Place the noisemaker in the sock and tightly close the end of the sock with the string. Make sure to leave several feet of string free. Next, swing the buzzer around by the end of the cord. If you are standing away from the noisemaker, notice that the pitch changes. A longer string will make the change in sound more noticeable. If the change is difficult to hear, listen for a 'wawa' sound.

The change in pitch of the sound is made by the change in frequency of the sound. Ever notice that a car or train sounds different as it approaches and then moves past you, even though the speed is constant? When an object is moving, it changes the frequency of the sound waves around it. Something that is moving toward you, will have an increase in sound pitch due to the compression, or bunching up, of the sound waves (see picture?) Just as something moving away from you will have a lower sound, as the waves are expanding. This is called the Doppler Effect. The Doppler Effect was first discovered by Christian Doppler (1803-1853), an Austrian physicist and mathematician. The Doppler Effect can be seen in the environment in a number of ways. The Doppler Effect is also seen with light waves, but in this instance, it is color, not pitch, which changes. The color of light is correlated to what the wavelength of the light is. Larger wavelengths by motion away from the observer is called a red shift, and shorter wavelength towards are called a blue shift. Astronomers use this information to track stars, planets, etc in space. Doppler radar also uses the Doppler Effect. The radar emits a electro-magnetic signal that bounces off an object in the atmosphere. The radar can 'see' something as large as a thunderstorm or as small as birds. This helps the radar determine how fast objects are moving, such as tornadoes, or just the wind in general.

The frequency the observer hears is the speed of propagation of the wave, divided by its wavelength. F= v/wL So, if the wavelength decreases, the frequency must increase, and vice versa. After a few manipulations for the change in wavelength, the equation for the frequency observed by a stationary observer who is in front of an approaching source of noise is: F(of) = (v/(v-vs))F(s ) Where F(s) is the frequency of the source of the sound, v is the speed of the source, vs is the speed of the sound, and F(of) is the observed frequency in front of the source.

Example: A train moving at 25.00 m/s emits a whistle of frequency 200.0H. If the speed of sound in air is 343.0 m/s, find the frequency observed by a stationary observer in advance of the moving source.

Solution: The observed frequency, Fof, is the variable that we are looking for. The speed of the source, v, is 25.00 m/s. The speed of the sound, Vs, is 343.0 m/s. The frequency of the sound, Fs, is 200.0 Hz. Simply plug in the numbers to get your final answer of 215.7 Hz. Notice that this observed frequency is higher than the frequency emitted by the source.

There are different applications for this problem, including: the frequency of the source as it is moving away from the observer; the source is stationary, but the observer is moving; and when both observer and source are moving.

For more information, or the equations for the above scenarios, see your local physics book. Look under DOPPLER EFFECT.

References: is the U.S. government's official web portal to all federal, state and local government web resources and services.