Birth of the cool

Some of the coolest sounds in the world are coming from a laboratory in Pennsylvania. At the lab, two US researchers are working on ways to use sound to chill food. They have produced a sonic fridge that converts very loud sounds to directly cool a fridge containing ice cream. The researchers hope their work will end reliance on greenhouse gases.

The researchers have exploited the fact that sound waves travel by compressing and expanding the gas they are generated in. In the 1980s, Scott Backhaus and Greg Swift at the Los Alamos National Laboratory realised that this compression/expansion action could be used to cool and heat metal plates placed in the path of the sound wave. A temperature gradient can be generated by putting a stack of plates in the right place in a tube in which the sound wave is bouncing around. Some plates in the stack will get hotter and others colder. All it takes to make a refrigerator out of this system is to attach heat exchangers to the ends of the stack.

Now, Matt Poese and Steve Garratt of the Applied Research Laboratory at Penn State University, USA, are doing work on a chiller cabinet for ice cream. The research is being sponsored by ice cream makers Ben & Jerry's and Unilever. The pair is using enormously loud sounds to keep the cabinet cool. Humans feel pain when they hear sounds of 120 decibels, a level typically reached at a rock concert. The sounds pumped through the Penn State fridge reach 173 decibels. Were the fridge ever to crack open, the vast sounds generated within would not escape because the intense noise can only be generated in the pressurised gas locked inside the cooling system.

Prototypes of fully functioning acoustic fridges have already been built and one has even flown on the space shuttle. The research could also make refrigerators more reliable because a thermo-acoustic fridge has fewer moving parts, the researchers claim.

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