Active Noise Control

This sketch is by Peter Andrews

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