Noisy aircraft can make life miserable for people living near airports and concerns about increased noise can restrict airport operations and plans for expansion. As a result, aircraft manufacturers are keen on finding new technologies to reduce noise emissions — and this has led researchers in the US to develop a new lightweight material that takes an altogether different approach to absorbing sound.

Unlike traditional soundproofing materials, which absorb sound in structures that resonate at acoustic frequencies, the new “honeycomb” material dissipates sound by passing it through narrow tubes, the sides of which gradually absorb sound energy in a process called “viscous shear”.

The material was invented by Jason Nadler and colleagues at the Georgia Institute of Technology. According to Nadler, one of its key benefits is that it can absorb sound over a wider range of frequencies than traditional materials.

Most existing approaches to sound reduction work on the principle that a material is most efficient at dissipating energy when it oscillates at its resonant frequency. When sound waves enter foams and other cavity-riddled porous materials, waves of certain frequencies force the air in the cavities to resonate, dissipating energy. This method — the “Helmholtz resonator” — is well established in architecture to remove unwanted frequencies from buildings.

A major limitation with this approach is that it only works for certain frequencies, which are determined by the size of cavities. There is great practical difficulty in constructing a porous material that can accommodate the kind of frequency array emitted by an aircraft engine during take-off.

Shearing sound

The viscous shear method gets around this problem because it functions independent of frequency. It works due to the interaction of porous media with the air through which sound propagates. Sound waves are forced into the parallel tubes which make up the honeycomb-like structure where they shear against the sides losing energy through friction and compressive stresses.

Nadler described the process as “fundamentally different from traditional techniques that absorb sound using a more frequency-dependent resonance”. “It’s the equivalent of propelling a little metal sphere down a rubber hose when the sphere is just a hair bigger than the rubber hose”, he added.

To determine the optimum dimensions for his honeycomb structure, Nadler and colleagues began with a prototype made from ordinary capillary tubes physicsworld.com. “Classical analytical calculations showed us that certain geometric configurations at the micron scale minimize resonant effects and maximize absorption”, he told physicsworld.com.

’Super alloy micro honeycomb’

The next challenge was to find a material that could withstand the high temperatures and turbulence inside an aircraft engine. Nadler also claims to have developed the world’s first “superalloy micro honeycomb” using a nickel-based superalloy. The advantage for aircraft application is that the material is strong, sufficiently heat resistant and also very lightweight.

Acoustic metamaterial researcher Huanyang Chan of Hong Kong University of Science and Technology said, “Developing these new materials further will present interesting physics for people working with classical waves. The concept of the phononic crystal could be used in studying the properties of these materials.”