Kundt's Tube - How IT Works

How It Works

The tube is a transparent horizontal pipe which contains a small amount of a fine powder such as cork dust, talc or Lycopodium. At one end of the tube is a source of sound at a single frequency (a pure tone). 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.

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 resonance, this means its length is a multiple of the wavelength of the sound wave. At this point the sound waves in the tube are in the form of standing waves, and the amplitude of vibrations of air are zero at equally spaced intervals along the tube, called the nodes. The powder is caught up in the moving air and settles in little piles or lines at these nodes, because the air is still and quiet there. The distance between the piles is one half wavelength λ/2 of the sound. By measuring the distance between the piles, the wavelength λ of the sound in air can be found. If the frequency f of the sound is known, multiplying it by the wavelength gives the speed of sound c in air:

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.