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A refrigerator with the coolest sound.

Glassblowers have heard the sounds of cooling for centuries. Take a glowing hot glass bulb, join it to a cool glass tube, and listen for an ethereal hum. As the tube warms, it "sings," emitting a glorious tone.

In theory, that reaction can run in reverse. Sound, injected into a system at just the right power and pitch, can cause cooling.

Tinkering with such "thermoacoustic cooling," Steven L. Garrett and his colleagues at the Naval Postgraduate School in Monterey, Calif., have built a small refrigerator that cools its contents by blasting them with sound waves.

The idea is fairly simple: Loudspeakers at each end of a gas-filled, U-shaped tube emit alternating pulses of sound. The gas heats up as it is compressed and cools as it expands. To remove excess heat, a heat exchanger -- acting like a car's radiator -- pulls heat away from the tube, cooling the gas down. Using this system, Garrett's group has fashioned a cube-shaped unit that will become the prototype for a household thermoacoustic refrigerator, he reported this week in San Francisco at the annual meeting of the American Association for the Advancement of Science.

Garrett's efforts to cool gases with sound began at Los Alamos (N.M.) National Laboratory in the early 1980s and resulted in the production of a small test unit -- the Space ThermoAcoustic Refrigerator (STAR) - that flew in the space shuttle Discovery in January 1992. His latest unit, the ThermoAcoustic Life Sciences Refrigerator (TALSR), employs improvements specially tailored for domestic use. TALSR, which is 40 times more powerful than STAR, is similar in design and cooling capacity to a home refrigerator, cooling to 4 degrees C with 205 watts of energy; the freezer compartment chills to a frigid -22 degrees C.

The little fridge achieves that feat by blasting a sustained, deafening tone at 160 decibels - 10,000 times louder than a rock concert - into a mixture of inert gases. Yet, with virtually no moving parts (only the speaker driver), the sealed, heavily insulated cooler remains quiet on the outside.

"We need a stethoscope to tell if it's running," says Garrett.

With an approaching 1996 ban on the production of ozone-damaging chlorofluorocarbons (CFCs), used in most refrigerators, sound-driven coolers offer some appeal. Using no CFCs, they are simple, safe, and environmentally benign. In theory, they can cool down to liquid-nitrogen temperatures or chill a room by a few degrees. Potential uses include portable air conditioners, storage units for heat-sensitive vaccines cargo containers for tropical fruit, and cooling systems for computers.

Today, the sound-fridge is neither as cheap nor as efficient as conventional appliances. But Garrett contends that they could be "competitive in terms of cost and energy efficiency," and possibly available to consumers, within 2 to 3 years. So far, TALSR has met its most important laboratory test -- successfully chilling its designers' beers.
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Title Annotation:sound-cooled refrigerator
Author:Lipkin, Richard
Publication:Science News
Article Type:Brief Article
Date:Feb 26, 1994
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