The photograph shows periodic structures inside a cylindrical cavity. The cavity was made of a transparent tube into which fine powder was poured.
Under the influence of a standing sound wave, the powder particles were regrouped into periodic structures in the form of disks, covering the cross-section of the tube, similar to what happens in the famous Kundt tube.
The particles are grouped at the nodes of the standing wave, where the pressure is minimal. In the center of the empty gap there are antinodes, zones of high pressure.
A standing sound wave always forms inside the resonators.
Moreover, since there is a pressure gradient,
Pressure Gradient Elastic Waves
necessarily arise inside the resonators.
PGEWs transfer heat
from low pressure zones (from standing wave nodes)
to high pressure zones (to the antinodes).
This heat transfer reduces the temperature at the nodes and increases the temperature at the antinodes of the standing wave. Which, in accordance with the direct thermoacoustic effect, increases the sound volume.
PGEWs increase the temperature at the antinodes,
and convective heat transfer in the opposite direction brings the level of temperature separation to a stationary state.