= Kundt tube experiment
{c}
PHYWE's implementation rubs a $1.60\,\mathrm m$ rod supported $0.40\,\mathrm m$ from each end. Its end disc drives a dusty glass tube; a movable tuning slide selects a clear pattern. A ruler measures repeated positions and a <thermometer> records gas <temperature>. The two supports select a rod <wavelength> equal to its length.<Velocity of sound using Kundt’s tube by PHYWE SYSTEME GMBH>[pp. 1–2, Set-up and procedure, Fig. 2 and equations (1)–(3)]{ref}
The University of Iowa also documents an electronic version: a <loudspeaker> drives the tube and a <microphone> on a plunger supplies an <oscilloscope> readout. This permits a spatial scan of <sound pressure> without relying on the dust pattern.<3D30.60 - Kundt’s Tube by University of Iowa Department of Physics and Astronomy>[Demonstration 3D30.60, Procedure: speaker and microphone-plunger alternative]{ref}
\Image[/kundt-tube-drive-and-rod-modes.png]
{title=Two ways to connect a time measurement to an acoustic spatial pattern}
{description=Original functional schematic. Top: electronic drive, movable pressure probe and an independent position scale. Bottom: ideal free-end rod shapes with one centre support or two quarter-point supports. The upper blue curve denotes <sound pressure>; the lower curves show axial <displacement> along the rod, rather than bending. Apparatus proportions are schematic; the two rod modes are separate alternatives.}
{source=kundt-tube-drive-and-rod-modes.py}
For a proposed pressure-scan measurement, keep the drive <frequency> fixed and move a small <microphone> through the tube. Record its position and fitted sinusoidal amplitude after each move; repeat the scan in reverse to reveal drift. Locate several consecutive maxima, rather than estimating a <wavelength> from a single pair. Reducing the drive checks whether the inferred spacing is independent of <amplitude> in the <linear acoustics> regime. A drive-off record estimates electrical background. These are controls for the proposed measurement, not reported observations from the cited demonstrations.
A <microphone> and its access opening can perturb the field. End compliance, losses and transverse modes also challenge the one-dimensional description. Dust traces identify a repeating structure, but the grains are not fluid parcels: the equations below describe <sound pressure> and <acoustic particle velocity>, rather than a detailed grain-transport model. The <temperature> and gas composition must remain sufficiently uniform to interpret a single fitted <speed of sound>.
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