So what you do is get yourself a boundary, a solid surface, and propagate sound waves across that. Thermoacoustic cooling depends on establishing and maintaining a temperature gradient. Enclose this assembly in a well-sealed tube. Then apply sound. Design for a refrigerator that cools by sound.
Air is cooled by compression and expansion.
For demonstration purposes, a standard stereo speaker funneled to one end of the tube will do nicely. Hit the right frequency and the tube becomes a powerful resonator. Maynard sketches the configuration on a blackboard. But it all stays inside that rigid tube, reflecting back and forth, building up. The steady pulse sets the air in the resonator quivering.