Kundt's tube is an experimental apparatus used to demonstrate the properties of sound waves, specifically standing waves. It is named after the German physicist August Kundt, who created the device in the 19th century. The apparatus typically consists of a long, transparent tube filled with a fine powder (such as a mixture of cornstarch and sugar) or some other medium that can easily be disturbed by sound waves.
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A Kundt's tube turns an invisible standing wave into a measurable spatial pattern. A driven air column supports a longitudinal wave; measuring its frequency and the spacing of repeated nodes of a standing wave gives the speed of sound. The same drive can compare the wavelengths in a gas and a vibrating solid. August Kundt's 1866 paper introduced acoustic dust figures; its footnote explicitly counts successive nodes of a standing wave as half a wavelength.[ref]
Ueber eine neue Art akustischer Staubfiguren und über die Anwendung derselben zur Bestimmung der Schallgeschwindigkeit in festen Körpern und Gasen by August Kundt. 1866 publisher record; original p. 502 footnote on half-wavelength spacing
The attractive feature is the separation of measurements: a clock or frequency counter measures repetition in time, while a ruler measures repetition in space. Their product supplies a propagation speed. The pressure pattern and the fluid-motion pattern are different, however; identifying which one a detector responds to prevents a quarter-wavelength mistake.