By Dr. Eugene Patronis
When a plane wave tube is excited at its origin by a tightly fitting, oscillating piston as illustrated in Fig. 1 the resulting wave motion is that of a plane wave propagating in the direction of increasing z. In this motion, the acoustic pressure and the particle velocity are uniform over the cross-section of the tube and the particle velocity oscillates only in the z-direction. The phase velocity of this plane wave motion is independent of the frequency of excitation. This is not the case for an arbitrary type of excitation nor is it necessarily true when a compression driver excites a plane wave tube as the emerging wave front from such a device may have some curvature. In such instances, one must consider a more general solution to the wave equation consistent with the geometry of the plane wave tube.