Micro-honeycomb materials are enabling a new physics
of sound reduction.
research engineer Jason Nadler has developed a new microchanneled material that
reduces aircraft engine noise by wearing it down through a process called
viscous shear. (Georgia Tech photo by Gary Meek.)
Noise from commercial and military jet aircraft causes environmental
problems for communities near airports, obliging airplanes to follow often
complex noise abatement procedures on takeoff and landing. It can also make
aircraft interiors excessively loud.
To address this situation, engineers at the
Georgia Tech Research Institute (GTRI) are turning to innovative materials that
make possible a new approach to the physics of noise reduction. They have found
that honeycomb-like structures composed of many tiny tubes or channels can
reduce sound more effectively than conventional methods. The two-year project
is sponsored by EADS North America, the U.S. operating entity of EADS.
dissipates acoustic waves by essentially wearing them out,” said Jason Nadler,
a GTRI research engineer. “It’s a phenomenological shift, fundamentally
different from traditional techniques that absorb sound using a more
materials, such as foams or other cellular materials comprising many small
cavities, exploit the fact that acoustic waves resonate through the air on
various frequencies, Nadler explained. Just as air blowing into a bottle
produces resonance at a particular tone, an acoustic wave hitting a cellular
surface will resonate in certain-size cavities, thereby dissipating its energy.
An automobile muffler, for example, uses a resonance-dependent technique to
reduce exhaust noise. The drawback with these traditional noise-reduction
approaches is that they only work with some frequencies-those that can find
cavities or other structures in which to resonate.
A New Approach
involves broadband acoustic absorption, a method of reducing sound that doesn’t
depend on frequencies or resonance. In this approach, tiny parallel tubes in
porous media such as metal or ceramics create a honeycomb-like structure that
traps sound regardless of frequency. Instead of resonating, sound waves plunge
into the channels and dissipate through a process called viscous shear.
Viscous shear involves the interaction of a solid with a gas or other fluid. In
this case, a gas (sound waves composed of compressed air) contacts a solid (the
porous medium) and is weakened by the resulting friction. “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,” Nadler explained. “Eventually the friction
and the compressive stresses of contact with the tube would stop the sphere.”
This technique, Nadler added, is derived
from classical mechanical principles governing how porous media interact with
gases, such as the air through which sound waves move. Noise abatement using
micro-scale honeycomb structures represents a new application of these
principles. “You need to have the hole big enough to let the sound waves in,
but you also need enough surface area inside to shear against the wave,” he
said. “The result is acoustic waves don’t resonate; they just dissipate.”
In researching this approach, Nadler constructed an early
prototype from off-the-shelf capillary tubes, which readily formed a
low-density, honeycomb-like structure. Further research showed that the ideal
material for broadband acoustic absorption would require micron-scale diameter
tubes and a much lower structural density.
Creating such low-density structures
presents an interesting challenge, Nadler said. It requires a material that’s
light, strong enough to enable the walls between the tubes to be very thin, and
yet robust enough to function reliably amid the high-temperature, aggressive
environments inside aircraft engines. Among the likely candidates are ceramics,
metals and superalloys.
Nadler has developed what could be the world’s first superalloy micro-honeycomb
using a nickel-base superalloy. At around 30% density, the material is very
light (a clear advantage for airborne applications), strong and heat resistant.
He estimates this new approach could
attenuate aircraft engine noise by up to 30%. Micro-honeycomb material could
also provide another means to protect the aircraft in critical areas prone to
impact from birds or other foreign objects by dissipating the energy of the
For more information, visit www.gatech.edu.