Kundt‘s tube

The German physicist August Kundt (1839-1894) succeeded in physically representing sound waves as early as 1866.

Kundt‘s tube makes it possible to visualize standing sound waves in a glass tube. Cork dust in the tube is set into motion through an intense sound wave.

Thus, when the air within the tube is set into oscillation at the tube’s resonant frequency, troughs (with very agile air particles) and nodes (with barely moving air particles) develop at fixed positions along the tube.

Here, the cork dust was replaced by a special liquid:

In the wave tube, the movement of the invisible sound waves in the air is thus made visible.

  1. the sound wave generated by a speaker enters the tube
  2. at the end it is reflected by a flat surface, the so-called stamp.
  3. the returning wave has a so-called anti-phase only in certain frequency ranges.
  4. at the same time, incoming and reflected waves must have the same intensity.
  5. if the intensity and its phase relationship match, a standing wave is created.

In our experiment, this happens at the following frequency ranges:

  •  165     Hz   +  90-100% volume
  •  330     Hz   +  90-100% volume
  •  445     Hz   +  90-100 % volume

Superposition of waves

Further phenomena can be observed when several waves are superimposed.

Example: If you throw two stones into the water at the same time, you can see that the resulting circular waves penetrate each other without disturbing each other. This applies to all types of waves.

Waves permeate each other without disturbing each other' s propagation.

There is a technical term for the case of a superposition of waves of the same frequency: interference.

In such an overlay, areas of amplification and absorption/elimination are formed.

This is the result of adding the amplitudes of the individual waves:

Amplification

When two crests or two troughs meet (two identical or similar phases), an even higher crest or an even deeper trough is formed - the amplitude of the deflection is thus increased.

Absorption or elimination

If the crest of one wave meets the trough of the other wave (the phase shift between the waves is then an odd multiple of half the wavelength), the wave is reduced - the positive and negative amplitudes add up. If the amplitudes are equal, complete elimination occurs.