From Jet Engines, The Sounds Of Silence
The jet engine roar near airports is so loud that some communities have put strict limits on aircraft noise. The issue may soon be moot, however: Virginia Tech researchers are silencing jet engines by generating noise of their own.
Using sound against sound is not new. Antinoise systems in car mufflers, for example, cut out unwanted racket by generating sound waves of the same frequency and amplitude--but in opposite phase. Still, the Virginia Tech team is the first to quiet jet engines with the technique--cutting noise dramatically by up to 15 decibels. Using sensors, the system tracks engine rotation, which controls noise characteristics. A computer then generates a countersignal that is broadcast through acoustic devices inside the engine's air inlet.
Researchers are now trying to increase the frequency range to cancel out more noise. Several jet engine makers are interested in the work, funded by NASA's Langley Research Center, but commercialization is still a few years away.