The Doppler Effect

  • Introduction
  • Video & Results
  • Materials
  • Procedure
  • Preparation & Notes
  • Download Files


In this activity, students will,

  1. Listen to a buzzer, and establish that it has constant pitch.
  2. Stand in a circle and observe the Doppler fluctuations in pitch as the buzzer is swung in a circle.

The Doppler Effect is a perceived shift in pitch of a sound as it moves toward or away an observer. As a sound approaches the device (or person) detecting the sound, it's apparent frequency increases. When the sound departs, the apparent frequency is lower than the actual frequency of the sound. In this activity, the moving sound is a buzzer stuffed inside a soft toy that is swung in a circle. The observer are students sitting in a circle around the swinging toy.

doppler picture

Video demonstration of the Doppler Effect in a pool


2 Video demonstrations of the swinging bear

A longer video with a very nice description of the Doppler Effect




  • stuffed animal or ball
  • A small battery operated buzzer or build one with the parts listed below
  • Small buzzer setup (high pitch)
  • Larger buzzer (lower pitch)
    • 300- 500 Hz buzzer ($3.59 at Radio Shack catalog # 273-0053)
    • 4 "AA" battery pack with switch ($1.99 at Radio Shack catalog # 270-409)
    • 4 "AA" batteries
    • soldering kit and some electrical tape
  • Rope or string to tie onto bear. We used an old surfboard leash so that the bear was velcro strapped in and the other end could be strapped onto the person swinging the bear.

The Doppler Effect

  1. Have one student take the bear with the rope.
  2. Arrange the rest of the group sitting down in a circle around the center person with the bear.
  3. Turn the buzzer on.
  4. Students should verify that the sound is at a constant pitch.
  5. Have the center student swing the bear over their head in a circle.
  6. Students around the edge should listen for the change in pitch. Have them close their eyes and raise their hand when the bear is approaching and lower it when it is moving away from them.

Do you think that buzzer's pitch is changing while it's swinging?

Is everyone in the circle hearing the same pitch at the same time?

Assemble the Buzzer (9 Volt buzzer)

  1. Connect the 9 volt battery snaps (red wire) to the switch with solder. Use tape to reinforce.
  2. Connect the buzzer (red wire) to the switch with solder.
  3. Connect the buzzer to the battery snaps (black to black wires).


Assemble the Buzzer (3 Volt buzzer)

This assembly is larger and heavier. It may require are larger stuff animal; however the buzzer is in the 300- 500 Hz frequency range. The lower frequency may be easier on the participant's ears if the demonstration is going to be running for a period of time. This assembly also has the switch incorporated into the battery holder so there is one less component to connect.

  1. Connect the 4 "AA" battery holder ($1.99 at Radio Shack) to the buzzer ($3.59 at Radio Shack) (red to red, black to black) with solder.



Insert the buzzer inside the soft toy.

  1. Cut a hole into the stuffed animal or ball large enough to insert the buzzer. (optional: sew velcro onto the openings and finish off the edges to prevent fraying material)
  2. Create a space large enough to insert the buzzer assembly.
  3. If you have a buzzer or small alarm, place inside and close.
  4. Attach rope to the stuffed animal securely for swinging.




None at this time.