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Standing waves in tubes

The stationary waves set up by the vibrations of the air molecules within the tube are due to the sum of two travelling waves moving down the tube in opposite directions. One of these is the initial wave and the other its reflection from the end of the tube.

All air-filled tubes have a resonant frequency and if the air inside them can be made to oscillate they will give out a note at this frequency.

This is known as the fundamental frequency or first harmonic.

The animation shows the first three harmonics that are possible for standing waves in a tube closed at one end.

Notice that since there must be an antinode at the open end only odd numbered harmonics, the first, third and fifth etc., are possible.
© Keith Gibbs/John Bourne 2008