Doppler Effect Buzzer

Demonstration Kit


Use this simple Doppler Effect Buzzer to effectively demonstrate the principle of the Doppler Effect. Or use it to illustrate the concept that sound does not transmit in a vacuum.


  • Sound waves
  • Wavelength and frequency
  • Doppler effect
  • Vacuum
  • Sound transmission

Safety Precautions

Be very cautious of the surroundings before rotating the buzzer. Remove any objects that may obstruct the path of the buzzer when it is rotated. Students should stand 10 feet from the demonstrator. Inspect the bell jar for cracks or chips—do not use if it is cracked or chipped. The demonstrator and students seated near the bell jar should wear safety glasses.


Materials may be saved for future use.

Prelab Preparation

Demonstration 1. Doppler Effect

  1. Cut a length of string approximately 1½ meters long.
  2. Insert one end of the string through one hole in the front face of the battery holder and then loop the string around to exit the second hole (see Figure 1).
  3. Tie the string to form a small loop to secure the string to the battery holder. Check to make sure the knot is tight (see Figure 2).
  4. Clip the 9-volt battery into the battery holder.
  5. Double-wrap the rubber band widthwise around the battery, battery holder and buzzer as shown in Figure 2. Make sure the rubber band does not cover the hole in the buzzer. The rubber band will secure the battery and buzzer in place so they will not come loose when the buzzer is rotated.
  6. Clip one alligator lead onto one battery-holder terminal. The second lead should remain unattached until the demonstration is performed.

Demonstration 2. Buzzer in a Vacuum

  1. Clip a 9-volt battery into the battery holder of the electric buzzer assembly.
  2. Clip one alligator lead from the buzzer onto one battery-holder terminal. The second lead should remain unattached until the demonstration is performed.
  3. Set up the vacuum pump, vacuum plate and bell jar similar to the arrangement shown in Figure 3. Please follow the appropriate operating procedures specified for your vacuum pump.
  4. Make sure the vacuum system functions properly and that there are no leaks in the evacuated system. Use vacuum grease to seal leaks, if necessary.


Demonstration 1. Doppler Effect

  1. Clip the second alligator lead to the second battery-holder terminal. (The buzzer should produce a loud, high-pitched ringing sound.) Students should hear the loud ringing.
  2. Hold on to the string at a position that allows the battery and buzzer to hang approximately 75 cm below the hand.
  3. Wrap the excess string around your finger or wrist and tightly hold the string in your hand.
  4. Carefully and slowly rotate the buzzer in a circle above your head.
  5. Students will observe the buzzers pitch repeatedly changing from a higher pitch to a lower pitch as it rotates. Continue to rotate the buzzer until all the students have perceived the pitch changes.
  6. Rotate the buzzer faster and slower to adequately show the change in pitch.
  7. Discuss the concepts of the Doppler Effect with the class.
Demonstration 2. Buzzer in a Vacuum
  1. Place the small foam block onto the vacuum plate. Rest the battery and buzzer on top of the foam block with the buzzer facing up as shown in Figure 4. Do not cover the evacuation hole in the vacuum plate.
    {12742_Procedure_Figure_4_Top view}
  2. Connect the second alligator lead to the second battery-holder terminal. (The buzzer should produce a loud, high-pitched ringing sound.) Students should hear the loud ringing.
  3. Place the bell jar onto the vacuum plate to cover the buzzer. (The ringing should still be audible when the bell jar covers the buzzer.)
  4. Turn on the vacuum pump and allow it to run until all the air is removed from inside the bell jar. (Refer to the vacuum pumps operating manual for determining when a complete vacuum has been achieved.)
  5. Once a vacuum is established inside the bell jar, close the evacuated system (using a three-way valve or vacuum plate stopcock, if applicable) and turn off the vacuum pump.
  6. The sound from the buzzer will not be heard.
  7. Students may suggest that the buzzer actually turned off during the evacuation process.
  8. To show students that the buzzer is still functioning, slowly open the stopcock or three-way valve to allow air to gradually flow into the bell jar. The sound of the buzzer will progressively become louder as more air enters the bell jar. (This will confirm that the buzzer was ringing the entire time and that something else prevented students from hearing the sound.)
  9. Discuss the observations and theories with the class.

Student Worksheet PDF


Teacher Tips

  • If the buzzer is too loud, place a piece of transparent tape over the hole in the buzzer. This should muffle the sound of the buzzer.
  • Large, thick bell jars may significantly dampen the sound of the buzzer. This may make it difficult for students seated far away from the demonstration to hear the ringing before the bell jar is evacuated. For best results, use as small a bell jar as possible, and have students sit or stand near the bell jar wearing safety glasses.
  • Small, inexpensive bell jar substitutes include 1000-mL battery jars, or 500-mL, hard, plastic sample containers. Be sure to inspect the jars for cracks and test the bell jar under a vacuum prior to using it in front of the class. A cracked glass jar may shatter when it is evacuated.
  • A three-way valve provides the capability to completely close and maintain the evacuated system, which then allows the vacuum pump to be turned off. Operating vacuum pumps are loud, so it is critical to have the ability to turn off the vacuum pump while still maintaining an evacuated system. If the vacuum plate already has a stopcock then a three-way valve may not be necessary.
  • Two-stage vacuum pumps create the best vacuum. A single-stage vacuum pump may also be used, but this vacuum pump will not produce a complete “high-quality” vacuum. Therefore, the buzzer may be slightly audible after the pump has evacuated the bell jar to its capacity.


The first activity that may be performed with the electric buzzer is the demonstration of the Doppler effect. The Doppler effect occurs when there is a frequency shift due to the relative motion between the source of the wave pattern and an observer. If a sound source of known frequency travels in a straight line toward one individual and away from a second individual, both individuals will hear a sound of a frequency different from the “known” frequency (see Figure 5).

The individual in front of the moving source will receive the sound waves more frequently than they are actually produced by the sound source. Therefore, the sound that this individual hears will have a higher-pitch than what is actually being emitted by the sound source. The individual that observes the sound source moving away will receive the sound waves less frequently than they are actually produced by the source. This observer will hear a lower pitch compared to the actual pitch of the sound source. An observer traveling at the same speed and direction as the sound source will hear the true frequency of the sound because there will be no relative motion between the source and the observer.

As the buzzer rotates toward the students, the pitch will increase. As the buzzer moves away, the pitch will decrease. The demonstrator will not perceive much of a frequency shift because the relative motion between the rotating buzzer and the demonstrator remains the same.

All sounds originate from a vibrating object. A vibration is simply a rapid wiggling of an object. The rapid back-and-forth motion of a tuning fork is a familiar example of a vibration. When an object vibrates it causes the air molecules surrounding the object to move. The rapidly vibrating object compresses the air molecules together briefly, and when the object moves away from the air molecules, a less pressurized, low-density air pocket is created. This region of lower density and pressure is referred to as rarefaction (see Figure 6).
It is important to note that the air molecules do not necessarily travel away from the object. Instead, the vibrating air molecules transfer their energy to adjacent molecules, which then transfer it to other adjacent molecules in a “chain reaction” type sequence. This series of events repeats as a second pulse emanates from the vibrating object. The regions of compressed and rarefied air “travel” away from the object. When the vibration is smooth and continuous, like that of a tuning fork, a continuous pattern of compressed and rarefied air travels away from the vibrating object in a pattern known as a sound wave. When the sound wave reaches an individual's ear, the tiny hairs in the inner ear are set in motion and vibrate. These vibrations inside the ear are interpreted by the brain as sound.

A key characteristic of sound waves is that they only exist in a medium with molecules that can be set in motion and vibrate. Water and metals are excellent carriers of sound waves. Soft materials, such as wood or plastic, absorb vibrations and do not transmit sound waves well. A vacuum does not carry sound waves because there are no air molecules to vibrate.

In the second demonstration, students discover the importance of a medium, such as air, for the transmission of sound.

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