Physics 11–12 · Year 11

The Doppler effect with a swung buzzer and a phone frequency meter

Module 3: Waves and Thermodynamics (Sound Waves)

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

A source moving towards a listener crowds its wavefronts and raises the received frequency; moving away spreads them and lowers it.

What you need

Small piezo buzzer (about 2 to 4 kHz) in a padded ball on a 1.0 m string
Phone running phyphox Doppler effect or Audacity spectrum with a microphone, tape measure, stopwatch
Open space

How to do it

Record the buzzer at rest and read its frequency f from the spectrum.
Swing the buzzer in a horizontal circle at a steady rate; time ten revolutions to find the speed v_s = 2 pi r / T.
Record from a point in the plane of the circle; read the highest and lowest frequencies as the buzzer approaches and recedes.
Compare with f (v/(v - v_s)) and f (v/(v + v_s)) using v = 343 m/s.

What you should see

A 3000 Hz buzzer swung at 1.0 m radius with a period of 1.0 s moves at 6.28 m/s; the recorded frequency swings between about 3056 Hz approaching and 2946 Hz receding, a spread of about 110 Hz that is clear on the spectrum. The learner knows it worked when the measured extremes agree with the predictions within about 2 per cent.

What changes

What you change: speed of the source
What you measure: received frequency
What you keep the same: source frequency
listener position
air temperature

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The pitch changes because the source gets louder as it passes; loudness and pitch change for different reasons.
The source's own frequency changes; only the received frequency changes.

Safety card

Low riskLearners carry it out

Hazards

swung object striking someone
string breaking

Controls

3 m clear radius
check the knot before each run
swing at a steady rate of about one turn per second

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH11-10 PH11/12-4
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-02 PY-11WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities