The phrase "sounds cool" has taken on a whole new meaning, thanks to researchers at the Penn State Applied Research Laboratory. Acoustics professor Steven Garrett and his team have come up with a way to use sound waves to keep ice cream cold.

Garrett, research associate Matt Poese, and research engineer Bob Smith have developed an environmentally friendly compact freezer case that uses the principles of thermoacoustic chilling. The freezer was developed for Ben & Jerry's, which is funding the project. The device is called Bellows Bounce and can keep ice cream at -20°C.

Most refrigeration systems used a class of chemicals called chlorofluorocarbons (or Freon), until they were banned in 1996 over concerns about their effect on the ozone layer. Since then, hydrofluorocarbons have been the choice for refrigeration. These chemicals, however, are suspected in global warming.

The Navy has been looking for years for an environmentally friendly cooling system, according to Steve McElvany, program officer for Environmental Quality Programs at the Office of Naval Research. For 15 years, the Navy has funded research on the use of thermoacoustic chilling. One of Penn State's programs in this field, the Triton project, resulted in a prototype three-ton chiller used for distributed cooling on an aircraft carrier, according to McElvany. That research helped Garrett and his team learn how to use a high-efficiency linear motor to produce the acoustic energy required by the thermoacoustic components.

At its most basic, a thermoacoustic chiller like the ice cream freezer is a closed pressure vessel containing a porous medium called a regenerator or stack, two heat exchangers, and a source of acoustic energy. An optimized loudspeaker generates high-amplitude sound energy in an environmentally safe, inert gas, which is converted into cooling power.

An ice cream freezer uses high-amplitude sound to keep frozen confections icy cold. No headphones are required.

The sound waves cause parcels of gas to oscillate in the pores of the stack. As they contract under pressure, the gas parcels heat up and transfer some of their energy to one pore wall. When they expand during the second half of the oscillation cycle, they cool and draw heat from the other wall of the pore. As a result, the gas moves heat through the stack from the cold side to the hot side.

In the case of the Bellows Bounce, the acoustic energy takes the form of 190 or so decibels, a volume that couldn't be attained even by The Who with amplifiers set to 11, according to Penn State's Poese. The highest volume you could safely listen to at a concert is about 120 decibels. But there's no danger of people being deafened by escaping sound: Levels that high can only be reached in contained, pressurized gas.

The sound pressure is the input to the system. The required sound pressure level is dictated by the amount of heat that is to be removed from the load—in this case, the ice cream, according to Poese.

The loudspeakers don't have to play a range of frequencies or tones, because music isn't the goal here. So, they're operated at resonance or at the tones they produce by the natural free oscillation of the system. The Penn State team, with a commercial collaborator, has developed special loudspeakers that operate near their natural resonance frequencies and use metal bellows as the flexure seal that replaces the standard loudspeaker cones to compress the environmentally safe gas.

Pressurized helium is the current gas of choice, according to Poese, because it offers the highest thermal conductivity of the noble gases. Diatomic gases, like oxygen, don't yield as large a temperature change as helium. Those high-temperature changes allow for greater chilling efficiency.

The thermoacoustic chiller is quite small. The entire refrigeration core is contained within the bellows. "We have achieved proof-of-concept for making a compact chiller that has a volume which is substantially smaller than earlier thermoacoustic chillers," Garrett said.

The Ben & Jerry's prototype chiller is built and operating in the Penn State lab. Hopes are that the next generation of the device will cool to -40°C.

The thermoacoustic chiller, though still in its infancy as a technology, cools as efficiently as a home refrigerator, according to Poese. But, it's still far from being a commercial product.

Ben & Jerry's, which is owned by Unilever, is exploring the technology for use in stand-alone "impulse cabinets," those freezers that tempt you with frozen ice cream on a stick.

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