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The Anti-Music of the Aerospheres

Lots of powerful technologies have a serious drawback: they generate way too much noise. Consider the jet engine. We couldn’t live without it, yet its noise can make life hellish for those in neighborhoods near airports. Researchers have long sought to improve methods of dampening the noise generated by jet engines, industrial equipment, automotive systems, even household products. Now it looks like aerospace engineers at Georgia Tech have found a promising candidate in an unlikely place: ceramic beads that were originally made for heat insulation.

With funding from NASA Langley Research Center, aerospace engineering professor Krishan Ahuja and doctoral student Richard Gaeta, Jr., built an acoustic liner they could wrap around noisy equipment to test the sound-dampening properties of the spheres. The researchers filled this shroud-like liner with beads-originally made from conventional ceramic powders for heat-insulation applications-that varied in diameter from 1 to 5 millimeters and featured multiple pin holes in their eggshell-thin walls.

Ahuja and Gaeta hypothesized that the loosely packed, perforated beads, known as aerospheres, would reduce noise by providing tortuous paths for the sound waves passing through. They also speculated that using spherical beads of varied sizes would absorb a broad range of sound frequencies.

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They found that the aerospheres effectively blocked the noise generated by several types of jet nozzles that produce high-velocity, high-temperature air flow. In fact, the ceramic spheres absorbed both low and high frequencies at levels comparable to sound-absorbing materials such as acoustic foam and fiberglass. These materials, however, would burn or melt at the operating temperatures of these nozzles, which reach 1,000 degrees Fahrenheit. Other insulating materials such as ceramic wool and metallic honeycomb structures could survive extreme high temperatures, but are manufactured in preshaped forms, which would be difficult and costly to incorporate into many products. The beads, on the other hand, can readily be poured into almost any space.

If weight problems can be addressed, the ceramic-bead liner could be particularly well suited for abating the earsplitting engine noise produced by the High-Speed Civil Transport (HSCT), which is now being developed in the United States to allow commercial air travel at supersonic speeds. The propulsion system of the HSCT relies on a nozzle that allows extremely efficient mixing of air and fuel at temperatures of 2,000 degrees Fahrenheit-but it also emits deafening noise over a wide range of frequencies.

The cost of manufacturing ceramic-bead liners for jet aircraft noise absorption is not yet certain, says Joe K. Cochran, Jr., a professor of materials engineering at Georgia Tech and president of Ceramic Fillers, Inc. in Atlanta, which manufactured the aerospheres. But he adds that the performance benefits of the beads would outweigh their costs, at least for aerospace applications.

The ceramic beads may also prove useful as a noise absorber for industrial, automotive, and household equipment. For example, they could be poured into the bodies of pneumatic tools, automotive exhaust systems, and hair dryers after the products are molded.

The Georgia Tech researchers are also investigating the use of plastic spheres, which would be less expensive to produce than ceramic varieties. These could be useful in comparatively low-temperature applications, such as the walls in homes, hotels, and concert halls; certain compartments in aircraft and automobiles; and sound barriers in factories and along highways. It looks as though in the not-too-distant future, the spheres may be dampening many kinds of discordant music.

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