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The Power of Sound

Sound waves in "thermoacoustic" engines and refrigerators can replace the pistons and cranks that are typically built into such machinery

Steven Garrett, Scott Backhaus

Last February, a panel of the National Academy of Engineering announced the results of its effort to rank the greatest engineering achievements of the 20th century. Second and tenth on that list were two very successful heat engines: the automobile (and hence, the internal-combustion engine) and the refrigerator and air conditioner, heat engines operated in reverse. But these two pillars of modern technology share another, less flattering distinction: Both have inadvertently damaged the environment—by clouding skies with smog, spewing greenhouse gases or leaking compounds that erode the earth's protective blanket of stratospheric ozone.

Figure 1. Glassblowers can sometimes hearClick to Enlarge Image

Over the past two decades, investigators like ourselves have worked to develop an entirely new class of engines and refrigerators that may help reduce or eliminate such threats. These thermoacoustic devices produce or absorb sound power, rather than the "shaft power" characteristic of rotating machinery. Because of its inherent mechanical simplicity, such equipment may one day serve widely, perhaps generating electricity at individual homes, while producing domestic hot water and providing space heating or cooling.

How do these machines work? In a nutshell, a thermoacoustic engine converts heat from a high-temperature source into acoustic power while rejecting waste heat to a low-temperature sink. A thermoacoustic refrigerator does the opposite, using acoustic power to pump heat from a cool source to a hot sink. These devices perform best when they employ noble gases as their thermodynamic working fluids. Unlike the chemicals used in refrigeration over the years, such gases are both nontoxic and environmentally benign. Another appealing feature of thermoacoustics is that one can easily flange an engine onto a refrigerator, creating a heat-powered cooler with no moving parts at all.

So far, most machines of this variety reside in laboratories. But prototype thermoacoustic refrigerators have operated on the Space Shuttle and aboard a Navy warship. And a powerful thermoacoustic engine has recently demonstrated its ability to liquefy natural gas on a commercial scale.

That sound-powered equipment can accomplish these tasks seems almost magical—and rightly so: Arthur C. Clarke once remarked that "any sufficiently developed technology is indistinguishable from magic." Below we attempt to reveal the legerdemain and explain the simple physics that makes thermoacoustic machines possible.

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