Engineers use innovative approach to hush aircraft noise

September 30th, 2008 - 4:35 pm ICT by IANS  

Washington, Sep 30 (IANS) 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.

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