Physics 11–12 · Year 11
Beats from two tuning forks or two close tones
Module 3: Waves and Thermodynamics (Sound Waves)
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The idea
Two waves of slightly different frequency drift in and out of step, so their sum swells and fades at the difference frequency.
What you need
- Two 440 Hz tuning forks, a small lump of plasticine to detune one, rubber striker
- Two phone tone generators or a signal generator with two channels, Audacity with a microphone, stopwatch
How to do it
- Strike both forks together and listen: no beats when they match.
- Press plasticine onto one prong and strike again; count the beats in 10 s.
- Record the sound and measure the beat period from the envelope of the waveform.
- With tone generators set 440 Hz and 444 Hz, count beats per second and compare with the difference.
What you should see
Tones of 440 Hz and 436 Hz beat 4 times per second; the recorded envelope has maxima 0.25 s apart. Counting 40 beats in 10 s confirms the difference frequency, and the detuned fork's frequency is found as 440 minus the beat rate.
What changes
- What you change
- frequency difference between the two tones
- What you measure
- beat frequency
- What you keep the same
- equal loudness of the two tones
- microphone position
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Beats are a third note produced by the forks; they are the slow rise and fall of the combined loudness.
- The beat frequency is the average of the two frequencies; it is their difference.
Safety card
Hazards
- loud tones held close to the ear
Controls
- keep generated tones at a comfortable volume
- strike forks on rubber only
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.
Sources
The pages the author read to write this activity.