Kundt's tube
Kundt's tube is an experimental
How it works
The tube is a
The sound generator is turned on and the piston is adjusted until the sound from the tube suddenly gets much louder. This indicates that the tube is at
The detailed motion of the powder is actually due to an effect called acoustic streaming caused by the interaction of the sound wave with the boundary layer of air at the surface of the tube.[4]
Further experiments
By filling the tube with other gases besides air, and partially evacuating it with a vacuum pump, Kundt was also able to calculate the speed of sound in different gases at different pressures. To create his vibrations, Kundt stopped the other end of the tube with a loose-fitting stopper attached to the end of a metal rod projecting into the tube, clamped at its center. 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. Once the speed of sound in the air was known, this allowed Kundt to calculate the speed of sound in the metal of the resonator rod. The length of the rod L was equal to a half wavelength of the sound in metal, and the distance between the piles of powder d was equal to a half wavelength of the sound in air. So the ratio of the two was equal to the ratio of the speed of sound in the two materials:
Reason for accuracy
A less accurate method of determining wavelength with a tube, used before Kundt, is simply to measure the length of the tube at resonance, which is approximately equal to a multiple of a half wavelength. 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. The node actually occurs some distance beyond the end of the tube. Kundt's method allowed the actual locations of the nodes to be determined with great accuracy.
See also
- Chladni plates, another standing wave visualization technique.
- Rubens tube, demonstrates the relationship between standing sound waves and sound pressure.
References
- . Retrieved 2009-06-25.
- ^ Kundt, August (January–June 1868). "Acoustic Experiments". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. Vol. 35, no. 4. UK: Taylor & Francis. pp. 41–48. Retrieved 2009-06-25.
- ^ a b Poynting, John Henry; Thomson, J. J. (1903). A Textbook of Physics: Sound (3rd ed.). London: Charles Griffin & Co. pp. 115–117.
Kundt's tube resonance.
- ISBN 0-521-42969-2.
Further reading
- Hortvet, J. (1902). A manual of elementary practical physics. Minneapolis: H.W. Wilson. Page 119+.