New Acoustic Liner Material Absorbs Noise; Withstands High Temperatures

August 29, 1997

Tiny, hollow spheres developed at the Georgia Institute of Technology nearly 10 years ago for high-temperature insulation also offer competitive noise-absorption properties, recent studies show.

Researchers are using the spheres to create an acoustic liner material they believe has several advantages over existing materials, including its ability to withstand temperatures of more than 2,000 degrees Fahrenheit. Most other liner materials come in preshaped forms. The more versatile spheres could be poured into existing structures -- from the walls of homes, hotels and concert halls to the framework of aircraft and automobiles. 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," said 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."

Development and testing of this new wide-band acoustic liner material was funded by the NASA Langley Research Center.

Material Consists of Hollow Ceramic Beads 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 like 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.

Testing Shows Noise Reduction Ability To test the noise-reduction ability of liners made with the spheres, researchers conducted a variety of acoustic tests with an impedance tube, which is a standard tool for measuring sound frequencies. They collected their data via a computer and a two-channel signal analyzer that 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 in 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 that the new liner material can absorb both low and high sound frequencies at levels comparable to traditional bulk-absorbing liners like fiberglass, Kevlar and foam. These materials are not as malleable, however, and cannot withstand temperatures over 2,000 degrees Fahrenheit, Ahuja said.

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 said.

Future Research To Study Other Materials Future tests will seek 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. However, a manufacturing process to produce plastic spheres must be developed first.

Ahuja is being assisted by doctoral student Richard J. Gaeta Jr. in this research.

Georgia Institute of Technology

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