Next, consider a parcel of air right next to the wall of the tube. Something a little different happens here. Because the wall stays still while the parcel of air moves back and forth, the parcel will heat up the wall towards the closed end of the tube (the pressure maximum), and cool down the wall towards the open end of the tube (the velocity maximum).
Furthermore, there are many little parcels of air doing this. Think of a row of parcels along the wall. Each one heats up the bit of wall towards the closed end, and cools down the bit of wall towards the open end. All together, they act like a little bucket brigade, transferring heat energy along the wall.
Ordinarily this would be a minor effect, hardly noticable, because only a very thin layer of air is in thermal contact with the wall. We can make the effect much bigger by adding a chunk of porous material inside the resonating tube, so that ALL the air is in thermal contact with it. If the holes in this material are big enough that the air can flow freely through it, but small enough that most or all of the air is in thermal contact with it, the end towards the closed end of the tube will get hot, and the end towards the open end of the tube will get cold. It turns sound energy into a thermal gradient.
Figure 3. The stack takes on the temperature of the parcel of air:
hot (red) towards the closed end, and cold (blue) towards the open
end. Many more parcels (not shown) act like a bucket brigade,
shuttling heat energy along the stack.
This page maintained by Wil Howitt