Noise-cancelling Windows Halve Traffic Sounds
People living in cities with warm climates face a problem during summer months: keeping windows open for ventilation means letting in traffic sounds. A noise-cancelling device could solve this dilemma. Bhan Lam at the Nanyang Technological University in Singapore and his colleagues have created a device that can halve the noisiness of urban traffic, 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 fixed these to the security grilles of a typical window in Singapore in an 8×3 grid. These grilles are a common feature across South-East Asia, says Lam. He adds that the spacing of the speakers was dependent on the frequency of the noise that they wanted to cancel out.
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 Hz, with up to a 50 per cent reduction in loudness for sounds within this range. It isn’t 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 tuned specifically to cancel out the hum of aircraft engines”, says Lam. “The speakers the team used were only 4.5 centimetres in diameter – too small to cancel out noise at frequencies below 300 Hz. “A speaker needs to move a huge volume of air for low frequency sounds.”
The team placed the window in a replica room and played road traffic, train and aircraft noise from another loudspeaker 2 metres away. The frequency of most of the noise from traffic and passing aircraft ranges from 200 to 1000 hertz. Large trucks and motorcycles tend to generate sounds on the lower end of the range, while the majority of the sound from is around 1000 Hz.