Kundt's tube

The tube is a transparent horizontal pipe which contains a small amount of a fine powder such as cork dust, talc or lycopodium.

Kundt used a metal rod resonator that he caused to vibrate or 'ring' by rubbing it, but modern demonstrations usually use a loudspeaker attached to a signal generator producing a sine wave.

When it was rubbed lengthwise with a piece of leather coated with rosin, the rod vibrated longitudinally at its fundamental frequency, giving out a high note.

The problem with this method is that when a tube of air is driven by a sound source, its length at resonance is not exactly equal to a multiple of the half-wavelength.

[3] Because the air at the source end of the tube, next to the speaker's diaphragm, is vibrating, it is not exactly at a node (point of zero amplitude) of the standing wave.

Drawing from Kundt's original 1866 paper in Annalen der Physik , showing the Kundt's tube apparatus (fig.6 & 7, top) and the powder patterns created by it (fig.1, 2, 3, 4) .
A modern version of Kundt's tube experiment, used in a South American university physics class. Instead of a transparent tube with powder in it to reveal the nodes, this uses microphones mounted in the tube. The piston (right center) is moved back and forth. When the microphone's position is at the nodes of the wave the sound pressure goes to zero. The sound power from the microphones is recorded on the chart recorder (center rear) .