Kundt's Tube

The Kundt's tube experiment is an acoustical apparatus invented in 1866 by German physicist August Kundt. The goal of this experiment is to measure the speed of sound and show the standing waves. This experience is used to demonstrate longitudinal standing waves, the wavelength and acoustical forces.


This method consists to create a standing wave in a circular tube, a pipe. The big tube permits to measure the low frequencies whereas the little tube measures high frequencies. The big diameter (100mm) measures between 90 Hz and 1900 Hz, the other between 800 Hz and 6500 Hz. The pipe used has a diameter to 100mm, so it is a big tube. This pipe contains a fine powder. At the end of the tube, there is a loudspeaker which converts the sinusoidal signal in sound wave. The loudspeaker is the sound source for this experience because it sends especially accurate frequencies, called resonance frequencies. 

Two ways can be used for the Kundt’s tube: opened or closed. The tube could be closed by a piston. It should be insert in the tube without touching it. The difference between opened and closed tube will be explained in the next part.

A standing wave is present in a tube because there is one incident wave (send by the loudspeaker) and one reflexive wave. When the end tube is closed, the wave is reflexive by the piston. Otherwise, when the tube is opened, the reflexion results of the different impedance. The acoustic impedance is the measure of the opposition that a system flow resulting of an acoustic pressure applied of the system.

The Protocol 

To show the standing wave, the team decided to try the experiment of the Kundt’s tube. To observe the standing wave, the team followed this protocol: 

*  Take the tube (in glass or Plexiglas) and put some 
   substance in it, for example styrofoam balls.

*  Link the signal generator, the loudspeaker, the amplifier 
   and the power source to each other.

*  Spread uniformly the substance inside the tube,

*  Fix the tube into the support,

*  (Introduce the piston to the other end of the tube),

*  Generate signal with the signal generator,

*  Move the piston in and out of the tube till the resonance 
   is obtained.


When a sort of wave and feel, some vibrations would be observed, the resonance and the formation of the standing waves would be see.

 

On the spot of the loudspeaker, there is a node because the speaker’s membrane vibrates and constitutes a region of maximum displacement. A node is a point along a standing wave where the wave has a minimum amplitude (constant pressure). It is fixed. The opposite of the node is an anti-node, a point where the amplitude of the standing wave is a maximum. This occur midway between two nodes.

Experiments

During the project, an amount of experiments has been designed and executed. All the demonstrations of the experiment took place to research the right equipment and frequency to show the standing waves.