PURPOSE: To demonstrate standing waves in an air column.
DESCRIPTION: An air column in a tube is excited by a loudspeaker installed at one end of the tube. The tube is stopped by a moveable plunger. Hold the plunger fixed at a node and vary the frequency of the oscillator. Or keep the oscillator frquency constant and vary the position of the plunger to hit nodes and anti nodes at resonance that can be heard. The plunger is moved down the tube length and the standing wave pattern is heard by listening to the maxima and minima.
Because this is a closed tube the resonances exist at odd multiples of one-quarter wavelength.
This is a very dramatic demonstration, and is very effective in providing an introduction to standing sound waves. For elementary classes, the waves can be compared with the motion of the coils in a slinky spring with longitudinal standing waves. But in actual fact, the standing waves are rather more complicated.
EQUIPMENT: Kundt's tube with oscillator, as photographed.
Updated by Jun Qi in 4/5/2000