= Kundt's tube
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= Kundt tube
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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>.<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]{ref}
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.
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