Noise control: sound wave system cancels out drum of traffic
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 [that] corresponds to a halving of the perceived loudness.”
But, it seems, the tinkle of an icecream van might still have you reaching for the ear-plugs – Elliott noted the approach worked best when combating low-frequency sounds. The team said the system was effective between 300Hz and 1kHZ – a range that encompasses most traffic and train noise – although the small speakers limited the mitigation of very low frequencies.
The system was not a magic silencing switch but the approach could bring benefits, Elliott added. “If you can reduce that low frequency noise that really distracts you from whatever you are doing then that is obviously a help.”
The team pointed to research indicating the detrimental impact of noise pollution on health, with high levels linked to conditions including heart attacks. Natural ventilation, using the system, was another benefit.
Elliott said the system required further development but could enter production in five to 10 years. Most uptake was likely to be in hot and humid settings, such as in Singapore, where air conditioning was common and urban noise problematic. “Ultimately it is an energy-saving thing,” he said.