A microphone outside the window measures the incoming noise so that an array of two dozen speakers can cancel it out
An array of 24 speakers can quiet city noise by as much as closing the window (NTU Singapore)
By Theresa Machemer
JULY 14, 2020
Living next to a busy street or constant construction makes opening the window for fresh air a test of patience. How long can you put up with the discordant clashing of car horns and jack-hammers? But now, researchers are developing a window-mounted system that can cancel out some of the din.
The team has worked on sound cancelling devices for decades, mostly to control noise inside cars and some airplanes, Nicola Davis reports for the Guardian. The new device, detailed in a paper published on July 9 in Scientific Reports, treats the window as the source of the noise and uses an array of 24 small speakers to cancel out the incoming soundwaves, noticeably reducing the volume.
“The performance with the active control system is not too much worse than closing the window,” says study co-author Stephen Elliott of the Institute of Sound and Vibration Research at Southampton University to the Guardian. “What we measured in the room is about a 10dB reduction in the sound pressure level … subjectively [that] corresponds to a halving of the perceived loudness.”
To test their design, the researchers set up a window with the Anti-Noise Control Window system attached. About six feet away, they hooked up a loudspeaker to play recorded sounds of planes flying overhead, trains rumbling past and cars in traffic.
When a microphone placed outside of the window picks up the outdoor noise, the small speakers on the window emit “anti-noise” into the room, per Inside Science. That “anti-noise” is soundwaves with the opposite wave pattern to the incoming noise. That cancels out some of the din coming in through the window by as much as ten decibels, which is about the difference between a normal conversation and street noise.
Bhan Lam, an acoustics researcher at Nanyang Technological University in Singapore, tells the New York Times’ David Waldstein that turning on the window system is like flipping a switch on noise-cancelling headphones.
Some noises, like aircraft sounds, were too low to effectively cancel out. And high frequency noises like birdsong and human conversation also crept through the window. The array of speakers is best at cancelling out constant noises with a frequency between 500 and 1,000 hertz, like traffic and trains, Charles Choi reports for Inside Science. Lower sounds, with frequencies below 500 hertz, could only be cancelled out by larger speakers, which ruins the system’s goal of allowing an open window for fresh air.
“In places like Singapore, we want to keep the windows open as much as possible,” Lam tells the New York Times, in order to use less air conditioning, which is carbon-intensive, and to circulate fresh air into the space and prevent it from becoming stale. “I grew up in Singapore. It’s a small city with a lot of noise, so I have some motivation to solve this problem.”
Next, the team plans to conduct field tests with the devices in real-world settings rather than pre-recorded sounds, reports Inside Science. Ultimately, Elliott tells the Guardian, the team believes that the system will be most useful in hot, humid climates as a way to save energy that’s used for air conditioning, and it might enter production in about five to ten years.
In the meantime, they need to resolve another issue: aesthetic. “One complaint that we get is that it’s ugly,” Lam tells the New York Times. But for those who are looking for respite and a cool breeze, the device does the trick.