Making quieter roads

If you live near a busy road, you may have been kept awake at night by the odd car racing past, or a bus idling at a nearby bus stop. And it’s not a trivial problem. How busy a road is, traffic flow and vehicle speed can influence road noise. But what can be done about it? Ben McAllister spoke to acoustic engineer Laurent Galbrun from Heriot-Watt University in Edinburgh...

Laurent - There are surveys that talk about 125 million people that are affected by levels greater than 53 decibels in Europe. And within those 125 million, it’s 37 million that are exposed to levels greater than 65 decibels - which are quite high. Environmental noise guidelines from the World Health Organisation recommend not having levels greater than 53 decibels.

Ben - What does that sound like? What's a comparison there?

Laurent - Well for example, if you're talking to somebody just one meter away from the person, that would be roughly around 65 decibels. Remember however that obviously I'm talking about 65 decibel of conversation if you're fairly close to the person to whom you're talking. Now you would have roads further away typically from your home let's say. So you don't expect such high levels and you don't want to have such high levels.

Ben - Right. So that's 53 decibels is considered acceptable at your home, not at the site of the road itself.

Laurent - At the recipient, yes.

Ben - Right. Okay. And how much noisier can roads be than that?

Laurent - They're much noisier typically than that, especially in densely populated areas. In very busy roads you can be above 80 decibels.

Ben - Why is it a problem if it's louder than that? What are the problems associated with noise pollution?

Laurent - So it's fairly common for people living in noisy areas with large roads to be adversely affected in their sleep; and there are cardiovascular diseases, and there is now pretty strong evidence of, in particular, what is called ischemic heart disease. If you live in a noisy area, close to a noisy road, you basically are more susceptible to have ischemic heart disease. So there is an increase in the probability of you having that.

Ben - What are the approaches currently being taken to reduce the impacts of road noise on people?

Laurent - There is noise control engineering, which is basically about looking at what you can do at the source first. So for example making vehicles quieter. The road surface as well can also reduce noise levels; just the porousity of the asphalt will actually absorb the sound, so less sound reflected. You can do those things at the source and it tends be always the prefered approach, because if you reduce the level at the source, all the surrounding area will benefit from that. The next thing you can do is to reduce the noise along the transmission path. So let's say you put a barrier. Sometimes you might not even notice them, because often it's just soil that is put next to motorways - it’s fairly common. And ultimately you look at the receiver. Let's say the house, what can you do at your house not to hear road traffic noise, and solutions will be typically things like replacing single glazing with double glazing. You can design buildings so that you always have a quiet area; so you always have bedrooms, living rooms, all the living spaces on the quiet area; and instead you have corridors, storage spaces, bathrooms on the noisiest area. So the occupants are least affected.

Ben - Are there any limitations to this approach? I mean why can't we just do this everywhere?

Laurent - The fact is that sometimes you can do as much as you can, but it's still not quite enough. Now especially in very busy areas, city centers, there is just so much that can be done, and that's where you start thinking, OK, maybe the solution here is not about just reducing noise levels. It's about using sound as a resource rather than a waste - something that is good, that people like. And that's where you can use what we call positive sounds such as water sounds. These can be used to actually mask the road traffic noise. And then you start basically focusing on the positive sound, and you finally end up just hearing really that, and block out the noise. Classical approach is that you would want to have the very similar frequency content to cover road traffic noise, that would be called energetic masking.

Ben - That is to say, if you’ve got a really low-frequency noise, like say a bus going past, you want something low-frequency to try and cover that up.

Laurent - Exactly.

Ben - What might that be?

Laurent - You might need for example a very large waterfall, let’s say, that creates a lot of very large bubbles that create a lot of low frequencies. But ironically, from all the work I’ve been doing, you don’t need energetic masking - you don’t need to have the same frequency content. Because ultimately people are drawn naturally to the sound they prefer, and that's called informational masking. So you might have a type of sound that doesn’t match in terms of frequency characteristics the noise, but that doesn’t really matter, because people will block out the unpleasant sound and will focus only on the pleasant one. So that's where perception is really important, and a lot of acoustic research currently is looking at perception and going beyond just physical parameters. So there are some studies that are quite interesting because they show that when you add greenery, even if you don't reduce noise levels, there is a perception that the place is quieter. You are actually doing nothing from a noise control engineering point of view but you are changing the perception of the space, and you are making it more pleasant.