Demonstrate Doppler Effect in 7 steps | Fun STEM Activity

Whoosh! How a Spinning Buzzer Bends Sound 

That rising-then-falling sound of a passing siren? You can recreate it in your backyard!

Time: 15–20 minutes     Complexity: Beginner       Setup: Buzzer, string, phone, open space

Overview

You’ve heard it before — an ambulance siren rises in pitch as it approaches and drops the moment it passes. This isn’t your ears playing tricks. It’s a real, measurable change in sound caused by motion, and it’s called the Doppler Effect. It was first described in 1842 by Austrian physicist Christian Doppler, and today it powers everything from weather radar to medical ultrasounds to speed cameras.

In this activity, you’ll swing an electronic buzzer on a string, stand nearby, and record the sound on your phone — then analyse the actual frequency shift in a free audio app.

Materials Required to demonstrate Doppler Effect

What you need:

• A small electronic buzzer or a cheap battery-powered alarm (keychain alarm works perfectly)
• A piece of string, 80–100 cm long
• An open outdoor space (away from traffic noise)
• A phone with a free audio spectrum app (e.g., Spectroid or Spectrum Analyser)
• Optional: a second person to swing while you record

Step-by-Step Instructions to demonstrate Doppler Effect

1. Tie the buzzer securely to one end of the string. Switch it on.
2. Open your spectrum analyser app and note the buzzer’s baseline frequency while it’s stationary. It will show as a stable horizontal line.
3. Begin swinging the buzzer in a horizontal circle above your head at a steady speed.
4. Record the audio on your phone while the buzzer swings — hold it at arm’s length, at the same height as the spinning buzzer.
5. Play back the recording. Listen for the alternating rise and fall in pitch.
6. Open the recording in your spectrum app — you should see the frequency line ripple up and down in a wave pattern.
7. Try swinging faster. Does the pitch shift become more dramatic?

The Science Behind the Doppler Effect Activity

Sound travels outward from a source as rippling waves. When the buzzer moves toward you, it compresses the waves ahead of it — squeezing them closer together, making them arrive faster. Faster-arriving waves = higher frequency = higher pitch.

When the buzzer swings away from you, it stretches the waves behind it — spreading them further apart. Slower-arriving waves = lower frequency = lower pitch. The buzzer isn’t actually changing its sound. Your position relative to its motion is changing everything.

[ Image: Sound wave frequency Doppler shift diagram ]

�� Pro Tip: The faster you swing, the greater the Doppler shift. Try swinging at three different speeds and compare the frequency peaks on your spectrum app. You’ll see the ripple grow wider as speed increases!

Expected Results

A typical keychain buzzer emits around 2000–4000 Hz. While swinging, you should hear a clear wah-wah oscillation in pitch. On the spectrum analyser, instead of a flat line, you’ll see the frequency trace ripple rhythmically — higher as the buzzer swings toward you, lower as it swings away.

Fun variations to try:

• Have a friend swing while you walk slowly toward or away from the buzzer — does it change what you hear?
• Try the same experiment indoors and outdoors and compare the clarity
• Record a video with the audio and slow it down in a video editor to see the pitch changes more clearly

Conclusion

You didn’t just hear the Doppler Effect — you measured it. With a buzzer, some string, and a free app, you captured the same wave-compression phenomenon that helps meteorologists track storms, doctors image unborn babies, and astronomers map the expanding universe. The rising pitch, the falling pitch, the ripple on the frequency graph — that was physics happening in real time, right above your head.

What Did You Learn?

• Motion changes perceived frequency. The buzzer’s pitch didn’t change — your position relative to its motion did.
• Compression raises pitch; stretching lowers it. Waves squeezed together arrive faster and sound higher.
• Speed amplifies the effect. Faster swinging = bigger frequency shift = more dramatic pitch change.
• The Doppler Effect works for all waves. Sound, light, radar — the same principle governs them all.