Washington: Engineers are turning to innovative approach to hush aircraft noise and reduce environmental problems for communities near airports.

For instance, Georgia Tech Research Institute (GTRI) are relying on honeycomb-like structures to reduce sound more effectively than conventional methods.

"This approach 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 frequency-dependent resonance."

Most sound-deadening 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. 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, according to a GTRI press release.

Nadler's research 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 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."

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.