Figure 2.

Forward and backward traveling waves in a tube of length l and uniform cross-sectional area S. A sound source on the left (at x = −l) launches forward going waves in the tube, which are reflected at the termination on the right (at x = 0) to form backward traveling waves. Note that when the cross-sectional dimensions of the tube are small compared to the wavelength of the generated sound, the sound pressure and volume velocity is constant within any tubal cross-section (e.g. the red colored section in the middle of the tube), and we can describe the spatial variation in sound pressure and volume velocity within the tube in terms of a single spatial variable x, the position along the long axis of the tube (e.g. Eqn. 1).