Most current methods for cooling things down require the use of chemical refrigerants. Perhaps the most famous of these are CFCs, which destroy the ozone layer and were banned in 1996. Although the replacement gases don't affect the ozone layer, many of the new chemicals still affect the environment because they are potent greenhouse gases. But according to a presentation given this week at the Annual Meeting of the Acoustical Society of America in Cancun, Mexico, these cooling chemicals in your freezer may one day be replaced by harmless sound waves.

The "thermoacoustic refrigerator" described by Matt Poese and Steve Garrett of Pennsylvania State University's Applied Research Laboratory uses high amplitude sound energy to cool its interior. Whereas the racket at a rock concert measures about 120 decibels (dB), the new freezer uses sound levels as high as 173 dB, nearly 200,000 times as powerful (the decibel is a logarithmic unit). The idea of using sound waves for cooling has been around for about 20 years. But the Penn State team, with partial funding from Ben & Jerry's ice cream company, succeeded in substantially reducing the size of the cooling apparatus by employing specially developed loudspeakers that generate sound waves at just one frequency. "The coldest temperature we have achieved with this test rig is eight degrees below zero--well below the freezing point of water," Garrett says. Although the set-up is not yet ready to show up on your neighborhood ice cream parlor's countertop any time soon, Garrett notes that it proves the concept is viable. "What began as basic research on the fundamental connections between sound waves and heat transport, funded by the Office of Naval Research," he says, "is getting closer to providing an environmentally benign substitute for traditional engine and refrigeration technology."