By Donna Lu
Danil Roudenko / Alamy
People living in cities with warm climates face a dilemma during summer months: keeping the windows open lets in fresh air, but with it comes the sound of traffic. A noise-cancelling device now allows for natural ventilation without the accompanying racket.
Bhan Lam at the Nanyang Technological University in Singapore, and his colleagues have created a device that can halve the noisiness of urban traffic sounds, reducing the sound coming through an open window by up to 10 decibels.
To cancel out road noise, the researchers used 24 small loudspeakers and affixed these to the security grilles of a typical window in Singapore in an eight by three grid. These grilles are a common building feature across south-east Asia, says Lam. He adds that they selected the distance to space apart each speaker depending on the frequency of the noise that they wanted to cancel out.
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The team placed the window in a replica room and played road traffic, train and aircraft noise from another loudspeaker located two metres away. Most of the noise from traffic and passing aircraft ranges between 200 and 1000 hertz. Large trucks and motorcycles tend to generate sounds on the lower end of the frequency range, while the majority of the sound from motorways is around 1000 hertz.
The researchers spaced each speaker 12.5 centimetres apart facing outwards and programmed them to emit sounds at the same frequency of noise detected by a sensor placed outside the window.
The device was most successful at cancelling noise between frequencies of 300 and 1000 hertz, with up to a 50 per cent reduction in loudness for sounds within this range. It is not optimised for the noise from human voices, which have higher frequencies.
The effect is similar to the technology used in noise-cancelling headphones, which are often attuned specifically to cancel out the hum of aircraft engines, says Lam.
The speakers the team used were only 4.5 centimetres in diameter – not large enough to cancel out noise at frequencies below 300 hertz. “A speaker needs to move a huge volume of air for low frequency sounds,” says Lam.
Having larger speakers is a possibility, but risks blocking out too much of the view from a window. In future, the team plan to test a prototype in real-world experiments.
Journal reference: Scientific Reports, DOI: 10.1038/s41598-020-66563-z
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