Would seem to use the same principle as the free piston Stirling engine, with the sound acting as the displacer piston.
Stirling engines are/were used to power electronics in areas with no mains. They are not very efficient but are highly reliable and need little maintenance. They will run off just about any heat source. These days they've mostly been replaced by thermocouple generators or solar PV and batteries.
Yup, it definitely looks like an acoustic Sterling engine (no solid displacer). Does anybody have a drawing for this thing?
Looks like one of of the tubes he used is made of glass. Does it have to be? Is the stuff at one end of the glass tube steel wool?
That would be the regenerator material. That improves efficiency as it retards heat loss from the hot side to the cold side.
This unit seems to be delivering a fair bit of energy, seems you can draw a couple Watts off it without killing the resonance, which is pretty good.
There are lots of micropower Sterling designs out there, but this one delivers at least a bit of real power.
Jon