This April, a fire-hydrant-size apparatus called a thermoacoustic freezer made its debut at a Ben & Jerry's in Manhattan. The event was a relatively quiet affair, which was no small achievement: inside the core of the steel cooling unit, which was attached to a standard ice-cream cabinet, a loudspeaker emitted a 195-decibel screech to keep quarts of ice cream cold. From the outside, you could hear only a soft hum.  How did it work? The freezer is based on the principle that sound alters the temperature of the air it travels through. Sound waves oscillate, compressing and expanding in rapid cycles: compression causes the air to get warmer; expansion causes the air to get cooler. An ordinary conversation might cause the temperature to fluctuate within one ten-thousandth of a degree, but crank the volume inside the pressurized thermoacoustic freezer and you create much larger temperature spikes. The trick, then, is to capture the coolness of the expansion half of the wave cycle while wicking away the heat generated during the compression half. An ingeniously designed stack of tightly packed metal screens and heat exchangers in the freezer does just this. Repeat the trick over and over as the sound waves fluctuate thousands of times a minute, and you can prevent your Chunky Monkey from melting.  The freezer was created for Ben & Jerry's by a team of researchers led by Steven Garrett, a professor of acoustics at Penn State University. The goal was to devise a method of cooling that was less environmentally hazardous than using chemical refrigerants like hydrofluorocarbons -- which is why the freezer is filled with an inert gas like helium or argon. Garrett and his colleagues are now working to build at least 50 units for long-term testing. Ben & Jerry's, meanwhile, donated its patent rights to Penn State. The company is content to fill freezers rather than build them. RYAN BIGGE
Ryan Bigge article, in review of year's notable ideas and developments, on thermoacoustic freezer, which is based on principle that sound alters temperature of air it travels through (M)