Can Anti-Noise Silence A Highway?

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

Sam - I have an apartment with a terrace oriented towards a highway. Would it be possible to use anti-noise to take away the highway noise on the terrace? Alternatively, to install huge anti-noise speakers next to the highway to take away the noise for the entire neighbourhood?I have an apartment with a terrace oriented towards a highway. Would it be possible to use anti-noise to take away the highway noise on the terrace? Alternatively, to install huge anti-noise speakers next to the highway to take away the noise for the entire neighbourhood?

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. Whilst it’s appealing, its uses are more limited than you might hope. 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 the 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 had a peak, so that when you add the two together, the peaks and troughs cancel each other leaving you with flat, still water.

Izzie - Okay, so, that’s the physics of sound waves covered… But could 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 sometimes used in cars and in those active noise control headphones people use on planes are effective. You could wear a pair on your terrace! But try active noise control in a large space where sound can move around in all directions, and it does not work unless you have a huge number of microphones to sense the sound, and a similarly large 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 - Even if you could get it to work, your neighbours might not be happy. 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 be made worse! Finally, even if you could get it to work, it will always be much cheaper to build and maintain a noise barrier. One trick you might try if the highway isn’t too loud is to put on some music to hide the traffic noise.One trick you might try if the highway isn’t too loud is to put on some music to hide the traffic noise.

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 that it is hardly worth it.

Izzie - Plus, an angry neighbourhood mob doesn’t sound ideal either.  Next time, we’ll be cleaning up for Caitlin’s Question

Caitlin - I live in a studio apartment without an automatic dish washer and the topic of detergent consumption often comes up with my boyfriend after I've handwashed the dishes. The hypothesis is that washing dishes manually leaves some harmful detergent residue which we consume next time the dishes are used for eating or drinking. Furthermore, automatic dishwashers are safe because the detergent residue levels have been tested and are controlled. We are both scientists and his argument seems reasonable but I have been unable to find any solid evidence one way or the other. Are the residues from common manual dish washing (not containing antibacterial additives) significantly toxic to humans in the short or long term?