TutorForAll

Investigating Science 11–12 · Year 12

Testing the Doppler prediction with a swinging buzzer and a phone

Module 5: Scientific Investigations

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

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 toward a listener is heard at a higher frequency and moving away at a lower one; a swinging buzzer and a spectrum app turn Doppler’s prediction into a measured test.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

  • a swinging object striking people
  • cord breaking

Controls

  • padded foam ball
  • check the cord and knots before each run
  • keep everyone at least 3 m outside the circle

Note

No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.

What you need

  • a piezo buzzer of about 3 kHz on a 9 V battery, packed inside a foam ball tied to 1.0 m of strong cord (or a phone playing a 3000 Hz tone in a padded pouch)
  • a second phone running the phyphox Doppler or audio spectrum experiment, stopwatch, tape measure
  • an open space at least 6 m across

How to do it

  1. Measure the buzzer’s frequency while it is still.
  2. Swing the buzzer in a horizontal circle of radius 1.0 m overhead; count complete turns in 10 s to find the speed.
  3. Record the spectrum with the second phone held in the plane of the circle at least 3 m away; read the highest and lowest frequencies.
  4. Repeat at two other swing rates.
  5. Compare measured frequencies with the prediction and state the hypothesis tested and the type of investigation used.

What you should see

At 1.6 turns per second on a 1.0 m radius the buzzer moves at 10.1 m/s. With the speed of sound 343 m/s at 20 °C (a = √(γRT), γ = 1.40, R = 287 J/(kg·K)), a 3000 Hz source is heard at up to 3090 Hz approaching and down to 2915 Hz receding, a spread of about 175 Hz; the measured spread grows in proportion to the swing speed.

What changes

What you change
speed of the source
What you measure
highest and lowest frequency heard
What you keep the same
  • same buzzer
  • same radius
  • listener position in the plane of the swing
  • air temperature recorded

Common misconceptions

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

  • The sound gets louder, not higher, as a source approaches.
  • The source itself changes its frequency as it moves.
  • The effect needs the listener to move.

Curriculum references

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

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/investigating-science-stage-6-2017
  2. education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/investigating-science/Investigating-Science-Module-5-guide.docx
  3. instructional-resources.physics.uiowa.edu/3b4010-doppler-effect-buzzer-and-string
  4. phyphox.org/experiment/doppler-effect
  5. phyphox.org/experiment/audio-spectrum

All Concept Studio activities