The loudspeaker seen on the left of the image is electrically driven and will maintain a standing sound wave due to the resonance of the tube. The gas used inside of this tube is an inert gas, meaning that they are naturally occurring and harmless. The sound wave created interacts with the particles of this inert gas by moving them back and forth in between the solid parallel plates called the stack. This results in refrigeration because of basic thermodynamic processes taking place on the gas particles which change the temperature several times per cycle. Most of this temperature change comes from the compression and expansion of the gas due to the sound waves produced by the speaker. The other part of the temperature change is caused by the transfer of heat from the gas to the stack. As the gas particles move to the right they are compressed which results in a temperature increase. That heat is then transferred to the warmer side of the stack. Then as they move back to the left the gas is expanded and cooled and once there the particles absorb heat from the colder side of the stack making that side even colder. So each cycle of the resonator moves small amounts of heat from the left side of the stack to the right side resulting in a cooling effect.