Each little box measures 0.2 inch x 0.2 inch. The sound field sets up a resonant "standing wave pattern" consisting of waves of various "modes" of the box. The sound also sets up a mean flow inside the box: this is related to the phenomenon of "acoustic streaming", which is caused by sound interacting with the boundary layer . When the airplane reaches 0g, the particles are no longer pulled down to the floor by gravity, so they are swept up by the Streaming flow and swirl around in the box until they reach "stable" locations. The stable locations are at, or near, the locations where the sound pressure level is at a minimum. Note that these locations can form either flat "walls" (like the ones at the right of the picture) or complex, currved walls like the one in the foreground.
Pressure Contours at a constant plane seen when looking from the plan direction.
Red and Blure represent maximum pressure fluctuation. Green is the node, and thus the most stable region. (Walls of solid particles form along the green area.)
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Another more interesting pressure field that seems
to "die" out as you approach one side of the box. (at y=Ly=y dimension)
This is a combination of: 4 4 0 + 3 4 0