Air column resonance – measuring the speed of sound
PhysicsOscillations & WavesAges 16–17
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Sign in to playA glass tube holds water and a 256–512 Hz tuning fork vibrates over its open end. Drag the water surface to change the air column length L: the sound becomes much louder when L + e = (2n − 1)·λ/4, which shows on a loudness meter, a graph of loudness against L and the standing-wave pattern in the tube, and can be heard with the speaker button. Record two consecutive resonance positions to find λ = 2(L₂ − L₁) and the speed of sound v = λf.
Lesson: Practical: measuring the speed of sound with a resonance tube – standing waves in a pipe closed at one end
What it shows
A tuning fork held over a column of air closed at the bottom by water sets up a standing wave. The water surface is a displacement node and the open end is close to an antinode, so the column resonates when L + e = (2n − 1)·λ/4; the end correction e ≈ 0.6r allows for the antinode lying slightly above the open end. Consecutive resonances are half a wavelength apart, so λ = 2(L₂ − L₁) does not depend on e, and v = λf. The model uses v ≈ 331 + 0.6t m/s, a steadily sounding fork and one loss factor that gives each peak a finite width.
How to use
Choose the Tuning fork frequency and press Strike the fork. Drag the water surface in the tube, or use the Air column L slider and the −1 mm / +1 mm buttons, while watching the loudness meter and the graph; turn on the speaker button to hear it. At the loudest point press Record L₁, lower the water to the next loud point and press Record L₂. Compare your speed with theory, then change Temperature, Tube radius r or End correction.
Parameters you can change
- Tuning fork frequency 256 Hz, 320 Hz, 384 Hz, 440 Hz, 512 Hz
- Initial air column length 0.03–1.15 m
- Air temperature 0–40 °C
- Include end correction (e ≈ 0.6r)
- Inner radius of the tube 1–3 cm
Questions to explore
- Why does λ = 2(L₂ − L₁) give a more accurate result than λ = 4L₁?
- If the fork frequency doubles, how does the distance between consecutive resonance positions change?
- When the air is warmer, is the first resonance position deeper or shallower in the tube, and why?
