Stationary wave in a resonance tube
Another way of explaining the air vibration in a tube is to regard it as resulting from the formation of a stationary longitudinal sound wave. In the first position of resonance a node is formed at the bottom of the tube and an anti node at the top. It is easy to remember that a node is formed at the bottom, since the air in contact with the bottom cannot move, whereas at the top it is free to move to and fro with maximum amplitude.
In the second position of resonance an extra node and antinode are formed inside the tube. It is useful to compare longitudinal stationary waves in a tube with transverse stationary waves in a string (page 328). In both cases the nodes and antinodes are separated by a distance equal to one-quarter of a wavelength. Owing to the difficulty of representing longitudinal stationary waves diagrammatically it is customary to represent them symbolically by the use of transverse wave curves. This has been done in Fig. 29.7 (c).