Kundt's Tube

Location:

• Cabinet: Waves & Sound

• Bay: {A3 to A5}

• Shelf: "T"

Description:

Kundt's tube is used to find the velocity of sound waves in a solid brass rod. The brass rod, which is fixed at its center, vibrates at its natural frequency when rubbed with a rosined cloth. The cork diaphragm at the end of the rod then excites the air molecules in the glass tube. By adjusting the tube, standing waves are produced which activate some cork dust sprinkled along the length of the tube. The cork dust piles up at the displacement nodes which are separated a distance of l/2. From this measurement, the frequency of the sound waves can be determined. Since the rod is fixed at its center, its length represents l'/2 where l' is the wavelength of sound in the rod. Knowing the frequency (from the standing waves in air) and the wavelength l' in the rod, the velocity v' = fl' can be found.

• Equipment:	Location
  Kundt's Tube	(W&S) A3 to A5, Shelf "T"
  Chamois or soft cloth
  Bowl and Shaker of Rosin Powder
  2 Bench clamps	Rode and Tack Cabinet by Room 2241
  Meterstick	In Lecture Halls
  Extra Cork Dust	Stock Cabinet
  Extra Rosin Powder	Stock Cabinet

Setup:

1. Clamp the base of the apparatus to the lab bench so it doesn't move when the rod is being rubbed.
2. Check that there is enough cork dust in the tube, if not add some from stock.
3. Slide glass tube out and gently shake it back-n-forth so the cork dust is evenly distributed along its length of tube.
4. Make absolutely sure that the cork diaphragm at the end of the brass rod is NOT touching the inner walls of the glass tube. The rod will not resonate if it is in contact with the tube.
5. The tube may need to be adjusted, moved back-n-forth, to reach resonance.

Demonstration:

Shake some resin powder on the Chamois or soft cloth, and rub first 6" to 10" of the "end" of brass rod. When rubbing use short pulling or pushing strokes and if the cork diaphragm at the end of the brass rod is NOT touching the inner walls of the glass tube. The rod should start to sing. You will want to practice this before class to master your technique.

While rubbing, have a volunteer from the class Slide the glass tube back-n-forth until resonance occurs and the cork dust starts piling up at the nodes. Measure the distance between the piles of cork dust to give l/2. Since v in air about 33100 cm/sec, the frequency can be determined using f = v/l. The length of the brass rod is 4 feet or 122 cm, gives the value of l'/2 in brass. Why? The fixed point of the rod forces a node in the center, and the free ends are antinodes.

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