This amazingly simple engine runs at 700rpm heated only by a small candle.
Thermoacoustic hot air engines (Sonic heat pump and refrigeration or thermoacoustic heat pump and refrigeration) of which nearly all are thermoacoustic stirling engines is a technology that uses high-amplitude sound waves in a pressurized gas to pump heat from one place to another - or uses a heat temperature difference to induce sound, which can be converted to electricity with high efficiency, with a (piezoelectric) loudspeaker.
This type of heat pump or refrigerator has no ozone-depleting or toxic coolant and has few moving parts. A device consisting of a series of small parallel channels, referred to as a ‘stack', is fixed in place at a set location inside the tube. In a standing wave thermoacoustic engine, the pressure and velocity fluctuations through the stack are such that heat is given to the oscillating gas at high pressure and removed at low pressure; this satisfies Rayleigh's criterion  for self-sustained oscillation and by this process heat is converted into acoustic power. For thermoacoustic pumps, the process is reversed. By using thermal delays in the stack, this process approximates the highly-efficient Stirling Cycle, but without the cranks, sliding seals or excess weight found in Stirling engines. Ceperley (1979) 
Modern research and development of thermoacoustic systems is largely based upon the work of Rott (1980)  and later Steven Garrett, and Greg Swift (1988) , in which linear thermoacoustic models were developed to form a basic quantitative understanding, while commercial interest has resulted in niche applications such as small to medium scale cryogenic applications. The technology is also suitable for air-conditioning for homes, commercial buildings, vehicles and other cooling and heating applications.