Lightweight membrane can reduce in-flight aircraft noise

Researchers have developed membranes that can be incorporated into aircraft to drastically reduce the low-frequency noise that penetrates the cabin and can be unpleasant to passengers' ears.
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 airplane 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 - like the noise of an aircraft engine. Adding insulation materials to limit the noise would add significant weight to the aircraft, making it much less fuel efficient.

Researchers from North Carolina State University and Massachusetts Institute of Technology (MIT) have found a possible solution.
They have created a thin, lightweight membrane that covers one side of the honeycomb structure, like the skin of a drum. When soundwaves hit the membrane, they bounce off rather than passing through.

"This design is promising for making structures that are strong, lightweight, and sound-proof," said Yun Jing, an assistant professor of mechanical and aerospace engineering at NC State and senior author of a paper describing the work.
"It's particularly effective against low-frequency noise. 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," Jing said.

The membrane is made of rubber that is about 0.25 millimetres thick, and adds approximately 6 per cent 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," said Ni Sui, a PhD student in Jing's lab and lead author of the paper.
"It could make flying much more pleasant for passengers - particularly in helicopters," Sui said.
The paper is published in the journal Applied Physics Letters.