Video: Resonant frequency

In order to study the properties of a sound wave, the  German physicist  August Kundt developed a device that now bears his name.

A transparent tube, of known length, and filled with air, is stimulated by a loudspeaker with variable frequency. The opposite end of the tube remains open.

The primary result of interest in this experiment  was that it revealed that a wave cannot always be propagated in the tube.

For any given frequency, the movements of air molecules are likely to remain small and random.

But, what August Kundt was looking for above all were the very particular conditions that produced resonance.

He showed that for a certain frequency, which was a function of the length of the tube, a wave appeared.

This was a standing,  or stationary, wave, because it did not propagate.  The total picture makes one think more of a local vibration than a wave, but it actually is a wave – one  that is the sum of two progressive waves of the same frequency and amplitude, but moving in opposite directions.

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Standing waves have a very characteristic shape. They can be recognized by their  nodes of vibration (places where there is no motion)….

and their antinodes of vibration (places where the vibration has maximum amplitude).

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Also make note of the fact that that there are several resonant frequencies that produce standing waves that differ from one another.