Thermoacoustics is the field of study that combines sound and heat. Thermoacoustic devices use the Ideal Gas Law and the Second Law of Thermodynamics to transform heat into sound waves or to use the oscillations of sound to transport heat.

The core of every thermoacoustic device is the stack, a collection of closely spaced plates, an array of channels (right), or a forest of pins across which a temperature gradient is imposed. A critical temperature gradient is defined by ωa/cp, where ω is the angular frequency, a is the speed of sound, and cp is the specific heat of the gas. If the gradient in the stack is less than Tcrit, the device is a refrigerator, and it's an engine if the gradient is greater than Tcrit. The tube housing the stack is closed at the hot end.

The Engine

In an engine, thermal energy is converted into kinetic energy. The temperatures of the stack are maintained with input energy, and moving air is the output. It is convenient to think of the air as made up of discrete little "parcels" rather than a continuous medium. The air at the hot end of the stack is not as warm as the surrounding walls, so heat flows into the air parcel. As the air warms, it expands and the parcel moves towards the cold end. As it travels, it cools, but the surrounding stack is colder still, so heat travels from the air to the stack. Since the air is now cooler, it contracts, and moves back towards the hot end to repeat the cycle.

The Refrigerator

In a refrigerator, such as the one at right built by Clever Fellows Innovation Consortium, it's the reverse. The acoustic energy is provided and used to transport heat. At the cold end of the stack, the air is even colder than the stack, so heat is absorbed from the refrigerated space. The air parcel, pushed by the supplied oscillations, then moves towards the hot end. As it travels, it has no choice but to compact, since it is jamming towards the solid surface of the closed end. The temperature rises, and the parcel becomes even hotter than the surrounding stack. Therefore, heat is transfered into the stack, to be carried away by cooling water, and the air piece cools. Finally, the oscillations force the parcel to return to its original position, and the air cools as it expands. Each individual parcel of air only travels a small distance, but the hot end of one piece is the cold of the adjacent part, so the parcels form a "bucket brigade" to carry the heat out of the refrigerated space.

The Organ

The organ uses heat to produce acoustic energy, so it is an engine. Each organ tube has three basic components (right). In the center is the stack, sandwiched between the heat exchangers that create the temperature gradient. The hot exchanger is near the closed end and the cold is by the open. The final component is the resonating tube, which determines the frequency of the oscillations.

The velocity is zero at the closed end of the pipe, making a velocity node. The pressure remains at ambient at the open end of the pipe, so it is a pressure node, and therefore a velocity antinode. There is one quarter of a cycle between a node and antinode (for example between zero and one in a sine wave), so the fire organ tubes are quarter wavelength resonators.

In other pipe instruments such as the tuba or oboe, there is a net movement of air through the instrument in addition to the acoustic vibration. However, the air in the thermoacoustic organ simply oscillates back and forth. There is no place that the air enters or leaves the pipe, so the same air molecules remain vibrating within the organ.