Thin rubber membrane keeps a lid on aircraft cabin noise

Raleigh, North Carolina – Researchers from North Carolina State University and MIT have developed a membrane that can be incorporated into aircraft to drastically reduce the low-frequency noise that penetrates the cabin.

A summary of the study, published by the NC State University on 27 April suggests that the solution might help bounce off sound waves and block them from entering aircraft cabins.

“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.

Aircraft designs incorporate light-weight 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.

However, the honeycomb structures are not efficient in blocking low-frequency noise – such as 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.

The solution created by the researchers is a thin, lightweight membrane that covers one side of the honeycomb structure, like the skin of a drum. When sound waves hit the membrane, they bounce off rather than passing through.

“It’s particularly effective against low-frequency noise,” Jing added.

“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 0.25 millimeters 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 PhD student in Jing’s lab and lead author of the paper. “It could make flying much more pleasant for passengers –particularly in helicopters.”