An acoustic PLANE WAVE is a wave where the ACOUSTIC PRESSURE is the same everywhere in all planes perpendicular to the direction of propagation. One way to envision this with acoustic particles is that an undisturbed medium would be represented as many people standing shoulder to shoulder and front to back forming lines in a rectangular grid in a large field. If each person locks arms with the people on their right and left (to form lines) then planes are formed. If one line of people all take a step forward and then backward pushing into the next line of people, and if those people take a step forward and backward pushing into their next line of people, and if this process continues, then a plane wave has been formed.
One can also envision a plane wave with selected acoustic particles highlighted in a tube, as seen in this ANIMATION. A piston moves back and forth in one end of the tube pushing on a line of acoustic particles, which in turn push on another line of acoustic particles, and so on. Clearly the particles are displaced back and forth from an at rest or equilibrium position in the direction the wave is propagating, away from the piston.
We will talk about the speed of these sound waves in air a little later in this unit.