Noise-Cancelling Smart Window Blocks Street Din

During hot weather, it's nice to open a window to let in a breeze. Maybe not, though, if the window also lets in a cacophony from cars and trucks roaring past.

Street noise is a nuisance, a health hazard, and often cited as a reason to abandon the city for the quieter pastures. Why can't technology ease the problem? We have noise-cancelling headphones; why not noise-cancelling windows as well? Now, researchers in Singapore have created just such a thing for mockup room, and they are working on adapting their proof of principle to a real room.

The idea is simple: A sensor picks up a regularly repeating waveform, like the sound created by a rolling wheel or a turning propeller. Electronics characterizes the wave, generates a mirror image of it, and emits that second antiwave" from speaker, causing the two waves' peaks and troughs to cancel out.

Antinoise works best for frequencies above 300 Hertz and up to about 1000 Hz-so think about the rumble of traffic rather than the cracking of fireworks that has been plaguing some U.S. cities this summer. Antinoise also works best in limited spaces, where the wave and its antiwave are sure to meet up properly, as in the gap between a headphone and an ear. However, with careful engineering, the audio trick can help in an airplane's cabin and even in a car. In airliners, the antinoise is conveyed through special shakers" attached to the fuselage; in cars it's channeled through the existing sound system.

In a paper published today in the British journal Nature, researchers at Nanyang Technological University describe how an array of 24 small speakers placed in a window, together with a sensor, can generate an antinoise signal strong enough to cut the room's noise by 10 decibels as perceived by the human ear-that is, A-adjusted decibels, or dBA. That's about the difference between heavy traffic 90 meters away (60 dBA) and a quiet moment in a city (50 dBA).

Photo: NTU Singapore Prof. Woon-Seng Gan of Nanyang Technological University poses with the handmade apparatus. Miniaturization could make such arrays far less apparent to the eye.

It's a striking achievement to make wave and antiwave cancel out perfectly throughout an entire room. The key is that the noise all comes through a relatively small aperture-the window, explains Bhan Lam, an electrical engineer and the leader of the group.

In a way, we are treating the window opening as the noise source," he tells IEEE Spectrum. Effective control of the noise source will result in noise control everywhere in the room." He adds that simulations show that it ought to work no matter how big the room is.

There are two engineering tradeoffs. First, as you move the speakers further apart, the highest frequency they can cancel goes down. And as you make the speakers smaller, you reduce their maximum output power and their bass response. But if you really want to make the most of today's speaker technology, Bhan says, you can enlarge the window so that it can accommodate bigger speakers.

Years ago, noise from overflying airliners so ruffled people at the U.S. Open tennis tournament, in Queens, NY, that the city arranged to re-route air traffic to and from LaGuardia Airport for the duration of the event. Why can't antinoise do that job instead?

In an open space, if the noise source is far away-say, from an aircraft-it becomes a challenging problem," Bahn explains. This type of control is termed as spatial active noise control, and the research is still in the fundamental stage; only simulations have been reported thus far."