Kundt's tube is an apparatus invented in 1866 by German physicist August Kundt for measuring the speed of sound. It mostly consists of a transparent tube of adjustable length with powder in it. Sound is produced at one end of the tube, and the tube's length is adjusted until the sound becomes louder, indicating the tube is at resonance and the sound forms a standing wave. The powder then accumulates at the nodes of the standing wave, where is no vibration. The wavelength of the sound can be determined by measuring the distance between nodes. Using the c = λf formula, sound speed (c) can then be determined from the wavelength (λ) and the frequency (f) of the sound.
The image above shows a modern implementation. Today, Kundt's tube is mostly used for educational or for artistic purposes.
Also see our entry 1787 – Chladni Plates.