Cool sounds run freezer of the future

Prototype: A Penn State scientist is busy putting the finishing touches on a new device for Ben & Jerry's.

Medicine & Science

February 17, 2003|By Michael Stroh | Michael Stroh,SUN STAFF

UNIVERSITY PARK, Pa. - It's crunch time in Steven Garrett's laboratory - or, as is more often the case around here, Vanilla Heath Bar Crunch time.

This month, executives from Ben & Jerry's will arrive at Pennsylvania State University to see the result of an unusual collaboration between the Vermont ice cream maker and the 53-year-old acoustic scientist. The goal: to create a radical refrigerator that chills not with chemicals but with sound.

Garrett is a leader in the little-known field of thermoacoustics, the use of high-intensity sound to heat and cool. To keep ice cream frozen, he and his team have designed a loudspeaker that would make heavy-metal headbangers weep - so loud that a short blast, he says, could ignite human hair.

More important for environment-conscious Ben & Jerry's is what the device doesn't do - pollute.

Existing fridges and freezers use as refrigerants chlorofluorocarbons and hydrofluorocarbons, substances suspected of contributing to global warming. Concerned that its fleet of ice cream freezers might be worsening the problem, Ben & Jerry's and its corporate parent, Unilever, in March gave Penn State $500,000 and one year to develop a cleaner alternative.

Now, fueled by the occasional pint of their sponsor's product, Garrett and his team are frantically working to finish their prototype thermoacoustic freezer.

"We put on a tremendous amount of weight working for these guys," says Garrett, just back from picking up a few pints of Cherry Garcia.

If you didn't know there was a connection between sound and temperature, you're in good company: Isaac Newton didn't get it either.

In the 1687 edition of his landmark Principia, the great Sir Isaac incorrectly calculated the speed of sound as 968 feet per second. More than a century passed before French physicist Pierre Simon de Laplace figured out how Newton goofed.

Laplace realized that a sound wave compresses and heats the surrounding air. And he knew that as the air grows warmer, sound travels faster. So, in 1816 he factored in temperature and came up with 1,116 feet per second, the same value used for the speed of sound at sea level today.

It turns out that it doesn't take much sound to generate heat. Ordinary speech, Garrett says, warms the air by a ten-thousandth of a degree. The Rolling Stones in concert would do a bit better - squeezing the air enough with a searing guitar solo to warm it by maybe two-hundredths of a degree.

Garrett's device must generate far more sonic energy than the Rolling Stones. To keep a pint of Ben & Jerry's at minus 4 degrees Fahrenheit, the operating temperature of a standard ice cream cooler, the Penn State device must generate 196 decibels of sound.

Sound that intense is hard to appreciate. Exposure to 130 decibels - a jet engine at close range - can cause physical pain and permanent damage to the inner ear.

Garrett once performed an experiment with a siren capable of 165 decibels. Wearing earphones over earplugs, he used a pair of tongs to place a baseball-size wad of cotton in the sonic beam.

"Within 15 seconds you see a flash of orange, and it disappears," he says. If his hair had strayed into the sonic blast, it too would have burst into flames.

Garrett's thesis adviser, former Penn State acoustician Isadore Rudnick, conducted even more bizarre experiments with his 165-decibel siren in the 1940s - research that ranks among the most unusual ever published in the field.

One paper, titled "Sonic Laundering," examined the ability of high-intensity sound to clean soiled clothing - sort of an early attempt to Shout it out.

In another experiment, Rudnick exposed flies, roaches, mosquitoes, caterpillars and mice to the siren. "The sound," he reported in his 1947 paper, "proved lethal in all cases."

Rudnick - later known as Dr. "Rough-on-Rats" Rudnick among some students because of his experiments - determined that it was not the volume generated by the siren but the heat that killed his subjects. Sensors wired to mice, for example, showed that the animals' internal temperature soared from 98.4 to 140 degrees Fahrenheit. When he conducted autopsies on the mice, Rudnick found that their colon tissue had started to bubble.

Garrett stresses that ice cream junkies have nothing to fear from his cooler, which won't look any different from the ones operating in convenience stores today.

"I don't want people to think that if they buy one of these it's going to set their hair on fire," he says. "Our refrigerator will be quieter than your fridge at home."

That's because the heart of the thermoacoustic cooler - a metallic canister the size of an overgrown Thermos bottle - will be deep inside the device. One end of the canister connects to the ice cream box. The other is fitted with a souped-up speaker. Unlike the speakers most people have in their living rooms, this one is designed to play a single frequency: the equivalent of a low G on the piano.

"You could say it runs on music," Garrett says. "Very boring music."

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