waste heat, converts the resulting pressure back into another heat differential and uses a Stirling cycle to produce the cooling effect. Operation A thermoacoustic device takes advantages of the fact that in a sound wave parcels of gas adiabatically alternatively compress and expand, and pressure and temperature change simultaneously; when pressure reaches a maximum or minimum, so does the temperature. It basically consists of heat exchangers, a resonator and a stack (on standing wave devices) or regenerator (on travelling wave devices). Depending on the type of engine a driver or loudspeaker might be used to generate sound waves. In a tube closed at both ends, interference can occur between two waves traveling in opposite directions at certain frequencies. The interference causes resonance and creates a standing wave. The stack consists of small parallel channels. When the stack is placed at a certain location in the resonator having a standing wave, a temperature differential develops across the stack. By placing heat exchangers at each side of the stack, heat can be moved. The opposite is possible as well: a temperature difference across the stack produces a sound wave. The first example is a heat pump, while the second is a prime mover. Heat pump Creating or moving heat from a cold to a warm reservoir requires work. Acoustic power provides this work. The stack creates a pressure drop. Interference between the incoming and reflected acoustic waves is now imperfect. The difference in amplitude causes the standing wave to travel, giving the wave acoustic power. Heat pumping along a stack in a standing wave device follows the Brayton cycle. A counter-clockwise Brayton cycle for a refrigerator consists of four processes that affect a parcel of gas between two plates of a stack. Adiabatic compression of the gas. When a parcel of gas is displaced from its rightmost position to its leftmost position, the parcel is adiabatically compressed, increasing its temperature. At the leftmost position the parcel now has a higher temperature than the warm plate. Isobaric heat transfer. The parcel's higher temperature causes it to transfer heat to the plate at constant pressure, cooling the gas. Adiabatic expansion of the gas. The gas is displaced back from the leftmost position to the rightmost position. Due to adiabatic expansion the gas cools to a temperature lower than that of the cold plate. Isobaric heat transfer. The parcel's lower temperature causes heat to be