Kundt's tube is an experimental acoustical apparatus invented in by German physicist August Kundt   for the measurement of the speed of sound in a gas or a solid rod. The experiment is still taught today due to its ability to demonstrate longitudinal waves in a gas which can often be difficult to visualise. It is used today only for demonstrating standing waves and acoustical forces. 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. The other end of the tube is blocked by a movable piston which can be used to adjust the length of the tube.
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If the frequency of the sound wave, f is known, we can calculate the speed of sound as,. The apparatus consists of a long transparent horizontal pipe G, which contains a fine powder such as cork dust or talc. At the ends of the tube, there are metal fittings.
At one end of the tube, a metallic rod AB, of uniform radius having one or two meter length is introduced. This rod is clamped at the middle and carries a circular disc D, rigidly fixed at one end. The radius of the disc is slightly less than the radius of the glass tube. The rod is inserted a few centimeters inside the tube, without touching it. The other end of the glass tube is closed by a metallic piston, P.
The position of the piston can be adjusted by moving it in or out. The whole apparatus is tightly clamped on a table, so that there will be no jerks on the tube during the experiment. The tube is dried and fixed in position after spreading a thin layer of cork dust in it.
The length of the metal rod is equal to half the wavelength of the sound wave in the material of the rod. The disc begins to vibrate backward and forward. The air inside the glass tube is set into forced vibrations with the frequency of the wave emitted and stationary waves are produced by reflection at the piston.
On altering the position of the piston, a point is reached where an overtone of the air column coincides with the fundamental of the rod, and thus produces resonance within the tube. At this stage, the cork dust in the tube is violently agitated and collected as heaps at the nodes. The average distance between the successive heaps will be equal to half the wavelength of sound in air.
If l a be the mean distance between the consecutive nodes of cork dust in air and l r , the length of the metal rod, then,. If f is the frequency of vibration produced, the velocity of the sound wave through the rod and air is given by,.
Knowing the speed of sound in air, we can calculate the speed of sound in the rod. Cite this Simulator:. Kundt's Tube Apparatus.
Sound in air is propagated only by means of longitudinal waves; waves in which the particle's motion consists of oscillations back and forth in the direction of propagation. In a solid such as a metal rod, sound can be transmitted either by longitudinal or transverse waves. In this experiment, longitudinal sound waves will be produced in a metal rod and an air column. Using the properties of wave motion, the frequency of the sound and the speed of sound in the rod can be determined.