Spinning Whistle Doppler Demo
Ambulances do it with sirens; you can do it with a whistle and a string. Spin the sound source and your ears will catch physics in motion.
Use a large open space with nobody within several meters. Tie the whistle very securely and check the knot before each spin. Stop immediately if anything feels loose.
Materials
- A whistle (or a phone playing a steady tone)
- About 1 meter of strong string
- An open outdoor space
- A friend to listen
Procedure
Attach the whistle
Tie the string firmly through the whistle's loop. Tug hard several times to prove the knot holds.
Spin slowly first
Hold the string end and swing the whistle in a slow horizontal circle above your head. Have your friend stand a few meters away and listen.
Spin faster
Speed up to a fast, steady spin. The listener should describe exactly when the pitch sounds highest and lowest.
Map pitch to motion
Have the listener raise a hand when the pitch is highest. Check: is the whistle moving toward or away from them at that moment?
Try different tones
If using a phone app, try a low tone and a high tone. Is the pitch swing easier to hear on one of them?
What to observe
The pitch rises as the whistle swings toward the listener and falls as it swings away.
Faster spinning makes the pitch swing wider.
The effect is clearest when the whistle moves directly toward or away from the listener.
Why it happens
This is the Doppler effect: motion toward you squeezes successive wave crests together, raising the observed frequency (), while motion away stretches them apart and lowers it. The pitch is highest at the instant the whistle's velocity points straight at the listener — exactly when an ambulance siren sounds highest as it approaches. Astronomers use the same shift in starlight to tell whether galaxies are rushing toward or away from us.
Try variations
Record the sound on a phone and look at the waveform — can you see the frequency wobble?
Spin in a vertical circle and listen from the side: where does the effect vanish?