Can anti-noise silence a highway?

Wouldn’t it be nice to enjoy a peaceful afternoon on an otherwise noisy terrace? Izzie Clarke asked Trevor Cox from the University of Salford to crank up the volume on this acoustic enquiry…

Trevor - Active noise control, also known as anti-noise, is a process that gets rid of an unwanted sound by using a second sound that is specifically designed to cancel out the first. To get active noise control to silence the traffic you would need to produce a sound that is the inverse of the highway noise. When these two add together, the rumble of traffic is removed. Imagine the traffic noise is like a water wave at the seaside, the cancelling sound needs to have a trough where the traffic noise has a peak so that when you add the two together the peaks and the troughs cancel each other out leaving you with flat, still water.

Izzie - So that’s the physics of sound waves covered. But would it really cancel out a noisy highway?

Trevor - Reducing noise this way works very well in small confined spaces, and low to mid frequencies. This is why it’s used inside some cars, and also in those active headphones that you can buy to use on the plane. But try active noise control in a large space where sound can move around in all directions and it doesn’t work unless you have a huge number of microphones that sense the sound and a similarly large amount number of loudspeakers to create the cancelling noise.

Izzie - Not only is noise coming off in different directions, the frequencies would also range outside this ideal low to mid range making it more difficult to remove. Plus, it might create a few new problems…

Trevor - The loudspeakers pumping out the cancelling noise is increasing the amount of sound energy in the neighbourhood. There will be places, maybe your neighbour’s house, where the noise will actually be worse. Finally, even if you could get it to work, it will always be much cheaper to build and maintain a noise barrier, but all is not lost. One trick you might try is to try not to get rid of the noise but to hide it - put on some music on your terrace.

Izzie - So it looks like the sound is here to stay. We also put this on the Naked Scientist forum where Alan said…

Alan - You can buy such good active noise reduction headphones but it’s hardly worth it.

Izzie - Plus, an angry neighbourhood mob doesn’t sound ideal either.