Aircraft

Multifunctional load-bearing engine structures that absorb sound is a goal of research funded by EADS North America at the Georgia Tech Research Institute.

After almost two years of research, Georgia Tech has produced a nickel-based alloy that has a honeycomb-like structure that dissipates sound waves as they travel through the porous material. Now, the institute and EADS are discussing possible follow-on work looking at cheaper alternatives to the nickel alloy.

The nickel alloy was chosen as it is a common, robust, alloy that would enable the sound dissipation theories to be tested. The sound's dissipation is caused by viscous shear, in which oxygen and nitrogen molecules in the air carried by a sound wave suffer friction as they pass through the honeycomb's micron-scale diameter tunnels, reducing the pressure and, thus, the decibel level.

"The further work could also aim to look at how much noise reduction this could deliver [for the overall engine system]," says Georgia Tech research engineer Jason Nadler.

The strength of the metal honeycomb means that it could be a load-bearing structure while retaining its low density. The nickel alloy has a density of about 9 grams per cubic centimetre, but Nadler wants lower densities. "We are trying to reduce that [density] by 75% or 90%. That is the goal," he says.

Noise has previously been absorbed by foams and other cellular materials whose cavities dissipate a sound wave's energy. This process is limited by the need to have as many sizes of cavities as there are sound frequencies to be dampened.