PURPOSE: To demonstrate standing waves in an air column.

DESCRIPTION: An oscillator in the 1000-5000 Hz frequency range drives a loudspeaker at one end of a clear glass tube, with the other end stopped by a moveable plunger. Varying the frequency of the oscillator or the position of the plunger, one can obtain a series of standing wave patterns, which are made visual by the motion of cork dust in the bottom of the tube. The standing wave pattern is shown to large groups by placing the device on an overhead projector. This is a very dramatic demonstration, and is very effective in providing an introduction to standing sound waves. Examples of standing waves as seen using the overhead projector are shown below.

For elementary classes, the motion of the dust can be compared with the motion of the coils in a SLINKY spring with longitudinal standing waves (G3-28: SUSPENDED SLINKY). In fact, the standing waves are rather more complicated, similar to TEM patterns of electromagnetic waves in long cavities. Slides in our collection, taken from an AJP article (see link below), can be used to illustrate some of the details of these standing waves.

SUGGESTIONS: Click here for figures illustrating details of vibrations in a Kundt's tube, taken from: Robert A. Carman, Kundt Tube Dust Striations, AJP 23, 505-507, (1955).

REFERENCES: (PIRA 3D30.60) See also the classical version, H3-05: KUNDT'S TUBE - STROKED ROD. See Demonstration Reference File for further information and articles regarding these standing waves.

EQUIPMENT: Kundt's tube with cork dust oscillator on overhead projector.

SETUP TIME: None.