Top 15 Technologies
15.
Tiny spheres absorb noise at high temperatures
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A student pours spheres into an aerospace injector.
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Tiny, hollow spheres developed at the Georgia Institute of Technology (www.gatech.edu) nearly 10 years ago for high-temperature insulation also offer competitive noise-absorption properties. They can be used to create an acoustic liner material believed to have several advantages over existing materials, including its ability to withstand temperatures >1090 °C.
Most other liner materials come in preshaped forms. The more versatile spheres can be poured into existing structures—from the walls of homes, hotels, and concert halls to the framework of aircraft and automobiles. They can even be encased in a quilt-like fabric to make portable curtains and blankets for use in noisy factories or at roadway construction sites where permanent structures are unnecessary.
"They can be used for any situation where you need to cut down noise," said Dr. K.K. Ahuja, Regents Researcher in the GTRI Aerospace & Transportation Laboratory and a professor in the School of Aerospace Engineering. They can be used for the above applications as well as to quiet hair dryers, fan housings, pneumatic tools, and combustors. Development and testing were funded by the NASA (www.nasa.gov) Langley Research Center.
The patented material consists of hollow, ceramic spherical beads ranging 1-5 mm in diameter. They have eggshell-thin walls with multiple needle-size holes in their surfaces, and are produced by Ceramic Fillers, Inc. Dr. Joe K. Cochran, a professor of materials engineering, developed the original spherical shells—called aero-spheres. They are made of readily available powders such as alumina and mullite.
The initial design did not include the surface holes, but Ahuja correctly speculated that this change would improve the spheres' noise-absorption properties, improving the value of the finished product.
Laboratory testing determined the sound absorption properties of the spheres. Standard steel BBs and spheres without surface holes were studied, confirming that hollowness aids sound absorption. The new liner material can absorb both low and high sound frequencies at levels comparable to traditional bulk-absorbing liners such as fiberglass, Kevlar, and foam. However, those materials are not as malleable nor can they withstand the high temperatures. Other materials (e.g., ceramic wool, mineral wool, and some metallic honeycombs) can withstand high temperatures, but are produced in preshaped forms.
Researchers tested the liner material for aerospace applications in which it would have to stand up to high-velocity, heated air flow. The spheres were poured into a hollow shroud surrounding a noisy jet issuing from a nozzle. Far-field noise data measured in an echo-free chamber confirmed that the spheres can reduce noise in such an environment. Aerospace applications include adding these spheres in the combustion area of stealthy jets, in ejectors surrounding high-temperature engine exhausts, and in engine test cells, hush houses, and ground run-up facilities.
Future tests will seek to pinpoint exactly how the spheres reduce noise. Researchers also want to test thicker spheres, which are believed to be more durable, and ones made of lightweight plastic, which should be cheaper to produce. While such spheres might have some advantages where heat resistance was unimportant, a manufacturing process for producing them must be developed first.
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