Open-Pipe Resonance: Length Sets the Pitch

1 · Predict

An open-open air column (like an open organ pipe) resonates at a fundamental frequency set by its length. If you use a shorter pipe, does the resonant pitch go up or down?

• A shorter pipe resonates at a lower fundamental frequency.
• A shorter pipe resonates at a higher fundamental frequency.
• Pipe length doesn't affect the fundamental frequency.

2 · Set Up

1. Open the pipe-resonance preset and press Reset. The pipe is open at both ends, in 20°C air.
2. Enable the fundamental-frequency readout on the pipe.
3. Set the pipe length for each trial and record the fundamental frequency.

3 · Collect Data

Plot fundamental frequency f₁ (y-axis) against 1/L (x-axis) for your three trials.

4 · Analyze

1. For one trial, compute f₁ = c/(2L) using c = 331.3·√(1 + 20/273.15) m/s for 20°C air. Compare to the table.
2. An open-open pipe's fundamental has a full wavelength λ = 2L. Explain, using v = fλ, why f₁ = c/(2L) follows directly from that wavelength.

5 · Extend

1. A pipe closed at one end (like a clarinet) only supports odd harmonics and has fundamental f₁ = c/(4L) — half this experiment's frequency for the same length. Why might closing one end double the effective wavelength?
2. The speed of sound c increases with air temperature. If you played this same pipe outdoors on a much colder day, would its resonant pitch go up or down? Why do wind instruments need retuning as temperature changes?

The Physics Behind This Experiment

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