Standing Waves (Longitudinal)

A loudspeaker is connected to a signal generator the held over a large measuring cylinder. The length of the column of air can be altered by adding water to the measuring cylinder.

At particular frequencies the sound heard is very loud. The frequencies at which this happens depends on the length of the air column.

Longitudinal standing waves are being set up in the tube as the sound waves reflect of both the closed and open ends..

Again, this only occurs at certain frequencies. The loops on the diagram below show how the amplitude of vibration varies along the length of the tube. The air molecules in the tube vibrate along an axis parallel to the length of the tube.

This may seem complicated but there are a few simple rules:

• An open end must be an antinode

• A closed end must be a node

• For a tube with one closed end only odd harmonics (multiples of the fundamental) are possible

• For a tube with two open ends all harmonics are possible