Physics 11–12 · Year 11

Resonance in a closed pipe: speed of sound with a tuning fork (syllabus practical)

Module 3: Waves and Thermodynamics (Sound Waves)

Practical, model not builtLow risk

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The idea

A column of air closed at one end resonates when its length is an odd number of quarter wavelengths, which fixes the speed of sound from a known frequency.

What you need

Open tube about 0.6 m long and 30 mm across, standing in a water-filled cylinder at least 0.5 m deep and wide enough for the tube to slide freely, so the air column runs from about 0.1 m with the tube resting on the bottom to beyond 0.5 m as it is raised
Tuning forks of 512 Hz and 256 Hz with a rubber striker
Metre rule, thermometer for the air temperature

How to do it

Strike the 512 Hz fork and hold it over the open end; raise the inner tube slowly until the note is loudest and record the air-column length L1.
Keep raising the tube to find the second resonance L2.
Repeat each reading three times; the wavelength is 2 (L2 - L1), which removes the end correction.
Compute v = f x lambda and compare with the value for the room temperature.
Repeat with the 256 Hz fork: its second resonance (near 1.0 m) is beyond most school tubes, so use its first resonance plus the end correction found from the 512 Hz readings, and confirm the same speed.

What you should see

At 20 degrees Celsius the speed of sound is 343 m/s (ideal-gas value with gamma 1.4 and molar mass 28.95 g/mol), so for 512 Hz the air column plus its end correction equals lambda/4 = 0.168 m and 3 lambda/4 = 0.503 m; in a 30 mm tube (end correction about 0.3 of the diameter, 9 mm) the measured columns are near 0.159 m and 0.494 m, and L2 - L1 = 0.335 m gives lambda = 0.671 m and v = 343 m/s. The single-length method reads low because it ignores the end correction; the difference method removes it, and the learner knows it worked when v lands within about 2 per cent of the value for the room temperature.

What changes

What you change: frequency of the fork
What you measure: resonant air-column lengths
What you keep the same: same tube
same room temperature
fork held at the same height

Common misconceptions

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

The pipe makes the sound louder by amplifying the fork; the air column stores energy from the fork over many cycles at its natural frequency.
Resonance happens at every length if the fork is loud enough; only lengths near an odd quarter wavelength resonate.

Safety card

Low riskLearners carry it out

Hazards

glass cylinder tipping
striking the fork on the bench and chipping it

Controls

stand the cylinder in a tray
strike 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.

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH11-10 PH11/12-3
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