Noise-cancelling windows halve traffic sounds even when they're open

Authored by newscientist.com and submitted by JackGreen142
image for Noise-cancelling windows halve traffic sounds even when they're open

A grid of speakers facing outwards can play sounds that cancel out traffic noises Danil Roudenko / Alamy

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 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 motorways is around 1000 Hz.

Read more: Living near noisy roads could make it harder to get pregnant

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,” says Lam.

Having larger speakers is a possibility, but risks blocking out too much of the view from a window. The team plans to test a prototype in real-world experiments.

Journal reference: Scientific Reports, DOI: 10.1038/s41598-020-66563-z

groundskeeping on July 13rd, 2020 at 21:43 UTC »

I'm an acoustical engineer and would like to say that oftentimes saying that sound energy is "halved" is misleading because we measure sound on a logarithmic scale (decibels). If you were to halve the pressure caused by the sound it would be a reduction -6dB, this is a great reduction yes, but if the traffic is loud enough that may still not be enough to meet code.

Edit: I wrongly put a -10dB reduction instead of -6dB

cynddl on July 13rd, 2020 at 19:08 UTC »

The team placed the window in a replica room and played road traffic, train and aircraft noise from another loudspeaker 2 metres away.

Real-life experiments might be much more challenging. Noise-cancelling is very sensitive to wind picked up by microphones. I'm not sure this could reliably work for a real window.

Article here by the way: https://www.nature.com/articles/s41598-020-66563-z

jhansonxi on July 13rd, 2020 at 18:41 UTC »

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.

An application of active cancellation. An interesting idea but not particularly innovative.