Sonic refrigeration

Sonic or thermoacoustic refrigeration is a technology that uses high-intensity sound waves in a pressurised gas to pump heat from one place to another. Current research at Penn State University in the USA (2003) is being sponsored by ice-cream manufacturers. This type of fridge has no ozone-depleting or toxic coolant, and has few moving parts. The sound pressure level used in the Penn State prototype is 173 decibels, which is much higher than the level that causes pain to humans, 120 dB, but is said to be inaudible outside the refrigerator.

The principle of thermoacoustic refrigeration is that sound waves create regions of alternating compression (high pressure) and rarefaction (low pressure) in the gas. As long as the gas is thermally insulated from the surroundings, these changes in pressure also cause changes in temperature. A set of metal plates is arranged in the gas chamber so that some of the plates are exposed to high-pressure hot regions of gas, and the others to low-pressure cold regions. The heat from the hot plates is conducted to the outside of the refrigerator, while heat from the inside of the refrigerator flows into the cold plates. The net effect is that heat is pumped from the inside of the refrigerator to the outside.