Soundproofing
New acoustic liner material uses tiny spheres to absorb noise and withstand high temperatures.
by Amanda Crowell
THE WALLS OF HOMES, hotels and concert halls may soon ring only with sounds the occupants want inside, rather than the cacophony of noisy cities and suburban sprawl.
photo by Stanley Leary
A new acoustic liner material developed at Georgia Tech
may be added to walls of homes, hotels and concert halls, and
may also reduce noise in aircraft, automobiles,
factories and roadway construction.
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Thanks to a new acoustic liner material developed at the Georgia Institute of Technology, noise may also be reduced in aircraft, automobiles and even in noisy factories or at roadway construction sites.
Researchers created the acoustic liner material from tiny, hollow spheres developed at Georgia Tech nearly 10 years ago for high-temperature insulation. The new liner material has several advantages over existing materials, including its ability to withstand temperatures greater than 2,000 degrees Fahrenheit, researchers say.
Most other liner materials come in preshaped forms. The more versatile spheres could be poured into existing structures — such as the walls of buildings and the framework of vehicles. They could 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 aren't needed.
"It can be used for any situation where you need to cut down noise," says Dr. Krishan K. Ahuja, Regents researcher in the Georgia Tech Research Institute's (GTRI) Aerospace & Transportation Laboratory and a professor in the School of Aerospace Engineering. "It can be used for soundproofing residential and commercial buildings and quieting the exhaust systems of aircraft and automobiles. It also can be used to quiet hair dryers, fan housings, pneumatic tools and combustors."
The NASA Langley Research Center funded development and testing of this new wide-band acoustic liner material.
The material, which Ahuja has patented, consists of hollow, ceramic spherical beads ranging from 1 to 5 millimeters in diameter. They have eggshell-thin walls and multiple needle-size holes on their surfaces.
They were produced by Ceramic Fillers Inc., a company formed in the late 1980s by researchers in Georgia Tech's School of Materials Science and Engineering to produce and promote the spheres.
Dr. Joe K. Cochran, a professor of materials engineering, developed the original spherical shells — which he calls aerospheres — as an alternative for industrial and home insulation. They're made from readily available ceramic 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. Cochran believes such modifications can only improve the value of the finished product.
To test the noise-reduction ability of liners made with the spheres, researchers conducted a variety of acoustic tests with an impedance tube, a standard tool for measuring sound frequencies. They collected their data via a computer and a two- channel signal analyzer. The analyzer sent a broadband signal toward a collection of ceramic spheres through an amplifier, then to a speaker in the impedance tube. By examining the amount of reflection of the incident sound, they determined the sound absorption properties of the spheres.
Researchers checked their results for accuracy by testing other materials in the same manner, then comparing these results with data gathered by other researchers using other impedance tubes. They also tested standard steel BBs and spheres without surface holes to confirm that the hollowness and the holes aid sound absorption.
The results show 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. These materials are not as malleable, however, and cannot withstand temperatures greater than 2,000 degrees Fahrenheit, Ahuja says.
Other materials — such as ceramic wool, mineral wool and some metallic honeycomb structures — can withstand high temperatures, but also are produced in preshaped forms.
Researchers also tested the liner material to see how it would perform in aerospace applications, where 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 a high-temperature flow 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," Ahuja says.
Future tests will try to pinpoint exactly how the spheres reduce noise. Researchers also want to conduct tests with thicker spheres, which they believe will be more durable, and ones made of lightweight plastic, which would be cheaper to produce.
Plastic spheres would be less brittle than ceramic ones and could be used in applications without heated air flow, such as in buildings, hand tools and the inlet sections of aircraft engines. But researchers must first develop a manufacturing process to produce plastic spheres.
Doctoral student Richard J. Gaeta Jr. is assisting Ahuja in this research.
For more information, you may contact Dr. Krishan Ahuja, Aerospace and Transportation
Laboratory, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, GA
30332-0844. (Telephone: 770/528-7054)
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