Coming Soon: Quieter Cabins?

For designers of both airliners and private jets, hushing cabin noise is a key element in the quest to make the ride more comfortable for passengers. While engineers have made strides in this area over the past several decades, a paper published in the journal Applied Physics Letters notes that a team of researchers at NC State University and the Massachusetts Institute of Technology has developed a solution they say will greatly assist in the further soundproofing of aircraft cabins.

The work began to test a hypothesis on how to make a lightweight soundproof structure. “We realized that maybe this is something that can be used for airplanes, and it turns out its perfect,” said Yun Jing, an assistant professor of mechanical and aerospace engineering at NC State and principal investigator on the project.

The process concentrates on the lightweight honeycomb composite materials that are used for interior cabin structures. On their own, the materials are not effective at blocking low-frequency noise such as the hum of an aircraft turbine engine. Adding insulation to block it raises weight. The investigators discovered that by sandwiching a paper-thin rubber membrane between two layers of the honeycomb material, they could drastically reduce the amount of sound that penetrates the material.

The process will add roughly 6 percent to the weight of a standard honeycomb panel, Jing said, but he estimates the overall weight penalty on the airframe should be less. “It’s hard to estimate,” he said, “but I would think that the total weight gain is going to be below one percent.”

“[The process is] particularly effective against low-frequency noise,” said Jing. “At low frequencies–sounds below 500 Hertz–the honeycomb panel with the membrane blocks 100 to 1,000 times more sound energy than a panel without a membrane.” The results of the study suggest this could translate to a large improvement in cabin comfort. “In our experiment, we found that we can reduce the noise by about 20 to 30 decibels,” Jing told AIN. “Just imagine you are going from the cabin to your living room.” While the team used latex rubber for its low cost and availability, Jing noted that many other materials could be used depending on manufacturing and safety requirements. The team will explore those alternatives in future testing.

The initial tests were carried out on small samples in an impedance tube, a device that measures the sound transmission qualities of materials, and Jing and his team are awaiting further research funding to move up to larger-scale testing. Once funded, the process could yield commercial applications within five years, said Jing.

Despite the early stage of the research, Jing says the team has already fielded several preliminary inquires from composite panel manufacturers, private aircraft acoustic insulation providers and at least one major airframer.