Science 7–10 · Year 9
Doppler effect: a buzzer swung on a cord, recorded on a phone
Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area)
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 sound source moving toward a listener is heard at a higher frequency and one moving away at a lower frequency, because the waves are bunched up ahead of the source and stretched behind it.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- swinging ball striking someone
- cord breaking and the ball flying off
Controls
- 9 m clear radius, with spectators and the phone holder outside it (a ball that breaks free at 12 m/s about 2 m up can land about 8 m from the centre)
- check the knot and cord before each run
- padded ball only
Note
No hazardous chemicals or naked flames are used. Complete the school's risk assessment for the activity before the lesson; the NSW Department of Education Science safety and compliance page points to CSIS 1.7 (Risk assessment – a pre-requisite for risk control) for how to carry it out.
What you need
- piezo buzzer (2 to 4 kHz) with a 9 V battery and switch, mounted in a padded tennis ball or foam ball, 1
- strong cord 1.5 m tied through the ball, 1
- phone running phyphox Doppler effect or Frequency history, 1
- tape measure, 1
- stopwatch, 1
- outdoor space with at least 9 m clear in every direction from the learner swinging the ball
How to do it
- Switch on the buzzer at rest and record its frequency on the phone.
- One learner swings the ball in a horizontal circle of 1.5 m radius overhead at a steady rate, no faster than one circuit per 0.8 s; another counts 10 revolutions and times them to find the speed (circumference divided by the time for one circuit).
- The phone, on a stand or held 10 m from the centre of the circle at the height of the circle, outside the clear zone, records the frequency for 20 s.
- Read the highest and lowest frequencies from the trace and compare with the rest frequency.
- Repeat at a faster swing, still no faster than one circuit per 0.8 s, and compare the frequency spread.
- Calculate the expected shifts from the measured speed and compare with the recorded values.
What you should see
At rest the buzzer reads a steady frequency, for example 3000 Hz. Swung at 10 m/s (1.5 m radius, about 0.94 s per circuit) the trace rises to about 3090 Hz as the ball approaches the phone and falls to about 2915 Hz as it recedes, a spread of 175 Hz that widens when the swing is faster. The listener hears the pitch rise and fall once per circuit. The learner knows it worked when the trace oscillates about the rest frequency with the swing period.
What changes
- What you change
- speed of the buzzer (m/s)
- What you measure
- highest and lowest frequency recorded (Hz)
- What you keep the same
- same buzzer
- phone at fixed distance in the plane of the circle
- steady swing rate
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- The source's frequency itself changes as it moves.
- The pitch is higher because the source is louder when close.
- The Doppler shift depends on how far away the source is.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-WAM-01SC5-WS-05SC5-WS-06
- Australian Curriculum v9AC9S9U04AC9S9I04AC9S9I05
Sources
The pages the author read to write this activity.