Noise control: sound wave system cancels out drum of traffic – The Guardian

If the hum of passing cars and the clatter of trains drives you to slam windows shut on a hot day, a new noise cancelling system could be music to your ears.
Scientists have developed a sound control system that can be fitted on to an open window, allowing a breeze to waft in while sounds from outside are quietened.
Active sound control involves using a microphone to detect incoming sounds then using an array of loudspeakers to emit sound waves of the same frequency but opposing pressure pattern to essentially cancel out the invading noise.
We have proved that it is possible, using loudspeakers in a window, to significantly reduce the sounds that come through an open window, said Prof Stephen Elliott, of the Institute of Sound and Vibration Research atSouthampton University, and a co-author of the research, adding the effect was almost as good as shutting a window.
Elliott said his team had worked on active sound control for decades; such systems were used to control sound within cars and propeller aircraft. Honda, for example, produce a million cars a year with systems where they control the sound inside the cabin using the sound from the loudspeakers, he said.
Writing in the journal Scientific Reports, Elliott, and colleagues in Singapore and Japan, report how they mocked up a room with a two-panel sliding window. In the open section of the window the team affixed a rectangular array of 24 loudspeakers, each 4.5 cm in diameter, together with a microphone to pick up sounds from outside the room.
They then placed another loudspeaker two metres away from the window, and played sounds recorded outside an apartment building in Singapore, including jet aircraft fly-by noise and road traffic sounds. A collection of 18 microphones was placed within the room to detect the level of sound passing through the window.
The results showed the system was effective, they said. The performance with the active control system is not too much worse than closing the window, said Elliott. What we measured in the room is about a 10dB reduction in the sound pressure level subjectively

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