Researchers from North Carolina (NC) State University and the Massachusetts Institute of Technology have developed a membrane that can be built into aircraft to reduce low-frequency noise in the cabin.

Researchers have developed a membrane that is effective against low-frequency noise in aircrafts. Source: NC State University. Image credit: Yun Jing“This design is promising for making structures that are strong, lightweight and soundproof," says Yun Jing, assistant professor of mechanical and aerospace engineering at NC State and senior author of the paper, “A Lightweight yet Sound-proof Honeycomb Acoustic Metamaterial,” published in Applied Physics Letters.

Researchers say the honeycombed materials that make up the floor and ceilings of most airplanes and helicopters are bad at blocking low-frequency noise, such as the noise that comes from an aircraft engine. However, adding sounds insulation to these honeycombs would add weight to the aircraft and make it less fuel efficient.

The solution put forward by the researchers creates a thin, lightweight membrane that covers one side of the honeycomb structure. Acting like the skin of a drum, sound waves bounce off rather than pass through the membrane.

“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 adds about 6% to the overall weight of the honeycomb and is made of rubber that is about 0.25 millimeters thick.

“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 Ph.D. student in Jing's lab and lead author of the paper.