I ACTIVE NOISE CONTROL
Imagine living next door to Luciano Pavarotti. You love the big guy, but his full-
throated practice sessions are rattling the china. To silence the booming tenor, you
could do one of three things: ask him to practice his Puccini outside; wear earplugs
around your apartment; or put some rock music on your CD player, crank up the
volume, and drown out Pavarotti's voice. The last option wouldn't be smart,
obviously, since you'd only be creating more noise. But what if the sounds coming
from your CD were the acoustic mirror image of the sound waves coming out of
Pavarotti's mouth? Instead of doubling the amount of noise, they would actually
cancel it out, zap it from the air. What you'd be left with is peace and quiet. The idea
of stifling Pavarotti or any human voice by scientific means is a bit fanciful, of
course. But the theory behind it - something acoustics scientists call antinoise - is
not. In fact, some of the biggest electronics companies in Japan take the concept
seriously and, together with a few small American firms, are making "active noise
control" (ANC) devices to help quiet the daily din. These are small but sophisticated
sound systems that analyze noise digitally and then - almost instantaneously -
generate equal but inverse sound waves, or antinoise. A typical ANC device can
weaken a targeted noise by 10 to 15 decibels. For most industrial noise, that means
reducing the sound level by 50 to 70 percent.
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