The project was funded by the US military as a way to harness the waste heat produced by radar systems and power electronics in the field.
When heat is applied -- with matches, a blowtorch or a heating element -- the heat builds to a threshold. Then the hot, moving air produces sound at a single frequency, similar to air blown into a flute.Link (Via Complexity Digest)
"You have heat, which is so disorderly and chaotic, and all of a sudden you have sound coming out at one frequency," Symko says.
Then the sound waves squeeze the piezoelectric device, producing an electrical voltage. Symko says it's similar to what happens if you hit a nerve in your elbow, producing a painful electrical nerve impulse.
Longer resonator cylinders produce lower tones, while shorter tubes produce higher-pitched tones.
Devices that convert heat to sound and then to electricity lack moving parts, so such devices will require little maintenance and last a long time. They do not need to be built as precisely as, say, pistons in an engine, which loses efficiency as the pistons wear.