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Lightweight membrane significantly reduces in-flight noise

28 April 2015

Researchers from NC State and MIT have developed a membrane that can be incorporated into aircraft to reduce low-frequency noise that penetrates the cabin.

Researchers have developed membranes that can significantly reduce aircraft noise when inserted into the honeycomb structures used in aircraft design (image: Yun Jing, North Carolina State University)

"This design is promising for making structures that are strong, lightweight, and sound-proof," says Yun Jing, an assistant professor of mechanical and aerospace engineering at North Carolina State University (NC State) and senior author of a paper describing the work online in Applied Physics Letters.

Aircraft designs incorporate light materials with a honeycomb-like structure into their wings and cabins. It's the material that makes up the floor and ceiling of most aircraft cabins. The sandwiched honeycomb structure makes it strong, and the light weight makes the aircraft more fuel efficient.

But these honeycomb structures are very bad at blocking low-frequency noise - particularly that from the aircraft's engines. And adding insulation materials to limit the noise would add significant weight to the aircraft, making it much less fuel efficient.

NC State and MIT researchers have created a thin, lightweight membrane that covers one side of the honeycomb structure, like the skin of a drum. Sound waves incident on the membrane 'bounce off' rather than pass through.

"It's particularly effective against low-frequency noise," Jing says. "At low frequencies - sounds below 500 Hertz - the honeycomb panel with the membrane blocks 100 to 1,000 times more sound energy than the panel without a membrane."

The membrane is made of rubber that is about 0.25mm thick, and adds approximately 6 percent to the overall weight of the honeycomb panel.

"The membrane is relatively inexpensive to produce, and can be made of any material that does not impact the structural integrity of the honeycomb panel," says Ni Sui, a research student in Jing's lab and lead author of the Applied Physics Letters paper. "It could make flying much more pleasant for passengers - particularly those flying in helicopters."

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