Voices of Science

The noise of traffic, so there are surveys that show it as a major cause of disquiet among the population, a high percentage of people living in cities claim to suffer stress due to the traffic noise. It’s probably far more noticeable and disturbing to the general population than aircraft noise. Another area that I worked in was around tyre noise, tyres in contact with the road. This really started as an approach from Dunlop, working with what was then Rover cars, and they said that really they didn’t have good ways of designing the pattern on tyres. They had good techniques for designing the pattern in terms of making them work well in wet weather, so having good grip on wet roads, but not proper acoustic models of what would be a noisy tyre and a quiet tyre. And it really meant that they would need to build a tyre and test it, to know which would be the most unpleasant noise, and in some cases the shapes of the patterns on the tyres were driven more by the marketing people and what would look rather snazzy and fast and attractive, rather than a systematic design process. The shape of a tyre meeting the road, they come together in a sort of horn shape, and that produces a very large amplification of the sound, and you could easily get twenty dB amplification due to – the noise comes due to pushing the air out, but pushing air out into the horn shape has a big effect. And you could get big reductions by just handling how much the amplification due to the horn was, the width of the tyre, where you had the open passages from the tyre into the outside. So that was one element of it and then the other element is as you put a tyre with tread on it in contact with a rough road, what is the vibration that you produce around the tyre, because that in turn displaces air and that’s the acoustic source. So we had to combine this model of the displacement of the air with the amplification of its effect due to the horn, and put all that together into a model that could be used to characterise, predict the noise, from a given tread block, and rough road – statistics of the rough road surface. It then moved into what can we do about it, and that led into a new project where we were looking at some radical changes to tyres. Because unless we do something quite radical, if you’re just tweaking changes in the tread block, you can get reductions for some speeds, but it’s hard to get an overall reduction. Of course the other way in which you can change things for tyres is to have porous roads; rather than change tyre, change the road and there are already quiet roads available that do that. But the downside is because they’re porous you get water in them, and then when it freezes it breaks up the road surface. So although these roads are available, they’re in the market, you do sometimes run over roads where you suddenly think gosh, this is quiet, that’s usually a porous road, but they don’t wear well, and in cold winters frost will do a lot of damage, and they need very regular replacing. So it’s much more convenient if you can do it on the tyre.