Posted by Liz Ernst on Fri, Dec 03, 2010 @ 12:12 PM
On a typical day in an American suburb, the steady whoosh of traffic on a nearby freeway drowns out the rustling of leaves in the wind. From across the street comes the nagging whine of a leaf blower, accompanied every few minutes by the deeper roar of a jet taking off from the airport. The cacophony of noise in the modern world is annoying to many and literally enough to make some people sick. Fortunately, new technologies are emerging to combat noise pollution.
Quieter Airports Take Off
Sound levels are typically measured in decibels (dB). Humans hear sound within a limited frequency range, reflected in a value known as A-weighted dB, or dBA. According to community noise guidelines published in 1999 by the World Health Organization, for a good night’s sleep background sound levels should not exceed 30 dBA. In outdoor living areas, sounds above 50 dBA are annoying to humans. The Occupational Safety and Health Administration (OSHA) requires employers to provide workers with hearing protection if they are exposed to an 8-hour time-weighted average of 85 dBA or more. For those living or working near flight paths of major airports, the noise of aircraft taking off and landing can exceed 100 dBA.
Seven years after the passage of the National Environmental Policy Act of 1969, the Federal Aviation Administration (FAA) adopted the Aviation Noise Abatement Policy (ANAP), which among other things, sought to reduce aircraft noise at the source—the aircraft itself. Under ANAP, airlines have retired or replaced noisier aircraft in three stages. But while aircraft are now significantly quieter than they were a few decades ago, many airports have added new runways and increased the number of takeoffs and landings. And urban sprawl has resulted in more people living around airports than ever before. The result is continued public pressure to reduce aircraft noise.
The National Aeronautics and Space Administration (NASA) is spearheading research in reducing aircraft noise through its Quiet Aircraft Technology program. The FAA standard for aircraft noise is the EPNdB (or effective perceived noise dB—a measure that is weighted to reflect the particular range of sounds generated by aircraft). NASA aims to develop the technology to reduce commercial aircraft noise by 10 EPNdB by 2007 and another 10 EPNdB by 2019.
“Our goal is to provide the technology to contain all annoying aircraft noise within the airport boundary,” says Dennis Huff, chief of the Acoustics Branch at NASA’s Glenn Research Center in Cleveland, Ohio. “It will be up to regulations and the marketplace to decide when the technology is used before the noise reduction benefit is realized.”
Jet engine noise comes predominately from two sources. An approaching jet creates a high-pitched whine as the fan pulls air into the engine. As the jet passes by, a low-pitched rumble is created by exhaust leaving the engine.
Working with the major aircraft engine manufacturers, NASA has been able to reduce the former sound by designing engines with larger fans. Larger fan blades turn at a slower tip speed, which reduces both noise and fuel consumption. The turbo fan engines introduced in the 1970s are much quieter than the turbo jet engines they replaced, and engines being designed today are quieter still.
Different approaches are being used to reduce the noise produced by exhaust leaving the engine. Researchers have found that notching chevrons into the rim of the nozzle allows hot engine air to mix more thoroughly with the cooler ambient air. This decreases turbulence and reduces engine noise. Chevrons have been used so far on aircraft flown by America West and USAir. New engines with larger fans also slow exhaust air speed, for even more noise reduction.