Feb 122010
 
Microphone
Image by visual.dichotomy via Flickr

Opting for the quiet life

MANY of those who talk loudly into their mobile phones are just inconsiderate show-offs for whom no punishment is too evil. Sometimes, however, there is an excuse. Noise in the background can make it hard for your interlocutor to hear what you are saying. Raised voices are an inevitable consequence.

Soon, though, this excuse will vanish. Thanks to advances in manufacturing techniques, which allow miniature mechanical components to be built into electronic chips, it is now possible to add better noise-cancelling features to phones, and also to other products, such as the small “earbuds” used to listen to music players.

The idea behind active noise-cancellation goes back to the 1950s. Sound is a pressure wave in the air. By making an identical but inverted version of that wave and playing it through a loudspeaker, peak should fall on trough, and trough on peak. The two waves should thus, in theory, cancel each other out—leaving silence.

But the practice is hard. Unless the sources of the noise and the antinoise coincide, the wave patterns will not overlap properly and the effect will be lost. A good approximation to perfect overlap can, however, be made by playing the anti-noise as close to the ear as possible, so that the part of the noise which is actually doing the irritating, namely that going into the listener’s head, gets cancelled, even if the rest does not. The problem with this approach is that you have to pick up the incoming soundwave with a microphone, work out in an instant what the antisoundwave needs to look like, and then get ahead of the original soundwave by sending a signal to a loudspeaker that will broadcast the antisoundwave at precisely the moment the soundwave arrives.

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