Standing waves in a tube

A standing wave in a tube is produced by interference with the wave traveling down the tube and its reflection from the end.

The green line is a graphical indication of the amount of vibration of the air molecules along the tube, resulting in rapid pressure changes at the antinodes. The vibration is longitudinal, the molecules oscillate in a direction along the tube.

The individual molecules vibrate  back and forth in a direction along the tube. To make them vibrate an energy input has to be made, for example by a speaker, a tuning fork or by blowing across the end of the tube. Most molecular motion occurs at the antinodes with changes between compression and rarefaction. The least (zero) molecular motion occurs at the nodes.

Showing the motion of molecules when there is a standing wave in a tube
A standing wave in a tube

By blowing across a tube the fundamental note will be produced. The wavelength of the note will be 4 times the length of the tube.

Graphical representation of the fundamental note in a closed ended tube
The fundamental note in a closed ended tube

Using a tuning fork or speaker we can induce harmonic notes.

Graphical representation of the second harmonic in a closed ended tube
Second harmonic

A standing wave is possible in an open tube. There must be an antinode at each end. This is therefore a graphical representation of the fundamental note.

Graphical representation of a standing wave in an open ended tube.
Standing wave in an open ended tube.

A PDF copy of notes which can be easily downloaded and printed is available here:Waves - standing waves in a tube (2 sheets)

Because standing waves are about moving particles the video below provides a more complete explanation

 

There are two other pages on standing / stationary waves each with video lessons: