We'll continue to assume that something is making the air resonate, and add heat exchangers. We said before that the stack would take acoustic energy and turn it into a heat pump. Actually, that will only happen if the thermal gradient in the stack is shallower than the gradient of the parcel. In this case, the parcel picks up heat energy from the stack at the cold end of its travel, and gives heat energy back to the stack at the hot end.
In this figure, the stack gradient (dotted line) is not as steep as the gradient of the parcel, so there is heat flow (red arrows). The changes in temperature cause the parcel to trace out an elliptical loop shape on this plot, which can be compared to a Carnot cycle.
Figure 6. Low gradient, refrigerator or heat pump.
The stack temperature gradient (dotted line) is shallower than the
parcel gradient, so the parcel alternately heats one end of the stack
and cools the other.
This page maintained by Wil Howitt