ONCE upon a time there was a singing road. Every time a vehicle drove along it, the road sang a catchy tune, in stereo, that left drivers humming happily for days. But not everyone loved its song, and the people who lived nearby began to get irritated with the road for singing the same thing, all day, every day. Eventually they got so angry that they complained to the mayor, who issued an order to smother the road beneath a layer of hot asphalt. The road never sang again.
This story is not completely true. It turns out that the road – built in 2000 in a Paris suburb called Villepinte – was not totally silenced when it was resurfaced in 2002. Visit Villepinte today and, if you listen carefully, you can sometimes hear the faint 28-note melody created by tyres racing over the pattern of corrugations on its surface.
The musical highway was a bold attempt to turn the noise produced by a busy road into something pleasurable. Unfortunately, it only served to highlight a growing problem. While vehicle designers have worked hard to quieten engines and muffle exhausts, they have been less successful elsewhere. The whoosh, swish and zizz of rubber on asphalt or concrete now accounts for more than half the noise that vehicles create, and as road building and car sales continue to boom – particularly in Asia and the US – noise created at the road surface is turning into a global problem. So the next great challenge for automotive engineers is to learn how tyre and tarmac can be made to work together to keep the peace.
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The racket from a busy highway is more than a minor annoyance. According to the World 91ɫƬ Organization, exposure to noise from road traffic over long periods can lead to stress-related health problems. And where traffic noise exceeds a certain threshold, road builders have to spend money erecting sound barriers and installing double glazing in blighted homes. Houses become harder to sell where environmental noise is high and people are not as efficient or productive at work.
Economists at the European Commission have calculated that taken together, these factors cost 0.65 per cent of the EU’s annual gross domestic product – tens of billions of euros each year. To help tackle the problem, the EC is introducing a directive in 2007 which will limit the tyre noise a new car can generate.
Although studies such as one organised by tyre company Pirelli in the 1990s suggest there is little manufacturers can do to reduce noise from tyres, Ulf Sandberg of the Swedish road and transport research institute at Chalmers University of Technology in Gothenburg believes a reduction of up to 10 decibels is possible – reducing the amount of noise energy tyres produce by a factor of 10.
He has been studying the problem for 30 years and has just embarked on two new projects, both aimed at finding quiet tyres, and drawing on ideas first mooted at least 15 years ago. “The pneumatic tyre has been developed over the decades into one of the most sophisticated mechanical components one can imagine,” says Sandberg. But it is also very good at creating and amplifying noise.
As a tyre rolls along, bumps in the road surface set its tread vibrating. These vibrations spread through the tyre, setting the side walls juddering and creating sound waves that radiate outwards (see Diagram). The rubber blocks of the tread generate noise as they hammer down onto the highway and as air is squeezed out through the grooves between them. Still more noise comes from a “stick and snap” mechanism: as the tread pushes down on the road, it adheres slightly. Then as the wheel turns, the tyre pulls away creating a snapping sound. All these noises can be amplified by resonances in the tyre itself, in the air trapped in the tread pattern and between the tyre and the road. The result is an unholy racket that is surprisingly difficult to analyse (see “Ear to the ground”).
To eliminate as much sound as possible, engineers designed a radical wheel in the 1980s. Instead of the familiar fat pneumatic treaded tyre on a small-diameter hub, their wheel used 17 thick spokes attached to a wide metal rim with slits cut in it. Four bands of solid rubber were fixed around the rim.
Although this design never went into production, Sandberg believes it has great potential. With no tread pattern there are no rubber blocks that can stick, slap or vibrate. Air trapped between the rubber bands can escape silently through the slits in the rim. “It has already been demonstrated that this wheel has very large noise-reducing potential if designed properly, and its wet-friction and hydroplaning properties should be superior to pneumatic tyres,” says Sandberg. Of course, it would also mean no more punctures. But he doesn’t know whether the tyre can be made durable enough or how it will handle under real conditions.
Despite the potential pitfalls, several companies and organisations are keen to find out whether the tyre is practical and are funding Sandberg’s research. They include the Technical University of Gdansk in Poland, Finnish tyre manufacturer Nokian and Swedish car maker Volvo. Sandberg plans to test a prototype this summer and if he can prove the tyre is commercially viable, he hopes the design will be adopted.
Sandberg is also investigating another novel tyre. This one is pneumatic, but it too has no tread pattern. Patented in 1980 by an engineer called Roger Williams at the tyre maker Dunlop, it has a porous surface much like a sponge. This eliminates the sound of air squeezing through the grooves of a conventional tread. Instead air simply escapes through the spongy surface. Fighter, a Swedish tyre company, is producing the first porous design this month and Sandberg hopes testing will begin this summer.
Meanwhile, researchers in the Netherlands – one of the most densely populated countries in the world – are taking a completely different tack. Since quieter tyres will obviously only cut the noise of vehicles whose owners fit them, the researchers are working to develop techniques for silencing the roads themselves. “Unless we find more cost-effective solutions, we could be spending ¬2 billion on roadside noise barriers by 2010,” says Erik Vos, scientific manager of the government’s road noise innovation programme.
In an attempt to cut this cost, the Dutch government is spending more than ¬20 million over the next five years on research into quieter roads. By 2010 it aims to have reduced noise levels from the country’s road surfaces by 6 decibels overall.
Ard Kuijpers, a mechanical engineer at acoustics and vibration consultants M+P in Vught, Netherlands, has come up with one of the most promising, and radical, ideas. He set out to tackle the three most important factors: surface texture, hardness and ability to absorb sound, and the result is a road that can be rolled out.
The rougher the surface, the more likely it is that a tyre will vibrate and create noise. Road builders usually eliminate bumps on freshly laid asphalt with heavy rollers, but Kuijpers has developed a multi-step process that he thinks can create the ultimate quiet road.
His secret is a special mould 3 metres wide and 50 metres long that was originally designed to create huge rolls of asphalt for use as sea defences. Hot asphalt, mixed with small stones between 3 and 6 millimetres across, is spread into the mould by a rail-mounted machine which flattens the asphalt mix with a roller. When it sets, the 10-millimetre-thick sheet has a surface smoother than anything that can be achieved by conventional methods.
To optimise the performance of his road surface – to make it hard wearing yet soft enough to snuff out vibrations – he then adds another layer below the asphalt. This consists of a 30-millimetre-thick layer of rubber, mixed with stones between 6 and 11 millimetres across (see Diagram). “It’s like a giant mouse mat, making the road softer,” says Kuijpers.
The size of the stones used in the two layers is important, since they create pores of a specific size in the road surface. Kuijpers says the surface can absorb any air flowing through a tyre’s tread, damping oscillations that would otherwise create noise. The pores also absorb sound energy from the vehicle above – the growl of the engine, for example. And there is an extra advantage: the gaps aid drainage, which can make the road safer in wet weather.
Compared with the complex manufacturing process, laying the surface is quite simple. It emerges from the factory rolled, like a carpet, onto a drum 1.5 metres in diameter. On site, it is unrolled and stuck onto a concrete foundation with bitumen. Even the white lines are applied in the factory.
Kuijpers has found a way to reduce noise using an even more sophisticated technique. He has designed a sound-absorbing concrete base that contains flask-shaped slots up to 10 millimetres wide and 30 millimetres deep that are open at the top and sealed at the lower end. These cavities act like Helmholtz resonators – when sound waves of specific frequencies enter the top of a flask, they set up resonances inside and the energy of the sound dissipates into the concrete as heat. The cavities play another important role: they help to drain water that seeps through from the asphalt above. This flow will help flush out dirt and debris and keep the pores in the asphalt clear.
He can even control the sounds that his resonators absorb, simply by altering their dimensions. This could prove especially useful since different vehicles produce noise at different frequencies. Car tyres peak at around 1000 hertz, for example, but trucks generate lower-frequency noise at around 600 hertz. To adjust the sound absorption qualities of a conventional road surface, its thickness would have to be altered: the lower the frequency of the unwanted sound, the thicker the layer required. But employ Kuijpers’s resonators and you build a concrete module of the same overall thickness and vary the size of the resonator cavities. This makes it easier to control which frequencies the concrete absorbs. On large highways, trucks tend to use the inside lane, so resonators here could be tuned to absorb sounds at around 600 hertz while those in other lanes could deal with higher frequency noise from cars.
Kuijpers believes he can cut noise by 5 decibels compared to the quietest of today’s roads. He has already tested a 100-metre-long section of his road on a motorway near Apeldoorn, and Dutch construction company Heijmans is discussing the location of the next roll-out road with the country’s government.
The success of Kuijpers’s design will depend on how much it eventually costs. But one thing is certain: we may not have heard the last of the singing road. A few years ago, a large US entertainment corporation asked one of Sandberg’s American colleagues to create a corrugated road surface at the entrance to one of its theme parks – one that could play “zip-a-dee-do-dah” as cars drove over it. He declined. Perhaps he had learned the lesson from Villepinte: the only thing we will ever want to hear from a highway is the sound of silence.
Ear to the ground
Measuring the noise that tyres create as they move over roads is fraught with difficulty. To exclude other sounds such as engine noise or the whistle of airflow, researchers must get their microphones as close as possible to the point where tyre meets tarmac. At just millimetres above the road, this can be hazardous even at low speeds.
One solution is the Tire Pavement Test Apparatus at Purdue University, Indiana, which has been operating since 2002. It consists of a solid cylinder 3.6 metres in diameter, to which samples of road surface are fixed. A wheel with a tyre is attached to a large rotating arm which runs the tyre over the road surface. To eliminate extraneous noise, the entire machine sits in an acoustic chamber. Microphones near the tyre pick up the road noise.
Robert Bernhard, who is in charge of the facility, claims it is better than any equipment tyre manufacturers have. It took a year to fine-tune the measurement system to eliminate the noise of the motor, gearbox and belt drive, and it can run at up to 48 kilometres per hour, which is one revolution each second. However, this is the upper limit. At that speed the end of the arm is accelerating at 9 g. “We can’t take it any faster because of the centrifugal forces – bits might start coming off,” says Bernhard.
To measure the noise from tyres at higher speeds you have to leave the lab and go out onto the highway. So researchers at the Transport Research Laboratory at Crowthorne, Berkshire, in the UK have built Triton, a 7.5-tonne truck converted into what is probably the most unusual mobile recording studio ever constructed. Its onboard anechoic chamber can be lowered close to the road surface. Inside the chamber is an axle with wheel, tyre and seven microphones. These capture tyre noise while the anechoic chamber keeps out most of the extraneous sounds. Using this set up, the team has been able to measure noise from tyres on real roads at speeds of over 100 kilometres per hour.
Good vibrations
A small team of British researchers has discovered that the sound generated by a rough road surface doesn’t necessarily have to be bad news for those who live close by. The Transport Research Laboratory in Berkshire, UK, has created a rough road surface that makes roads safer without adding to noise levels.
Installed at Rowner Road in Gosport, Hampshire, in the UK, their subtly corrugated asphalt surface, named Rippleprint, sets up strong vibrations inside vehicles but makes no extra noise outside. Drivers crossing it feel the steering wheel judder, and instinctively slow down. The idea is that this will cut accidents and help reduce traffic noise.
Sure enough, the accident rate for this residential road has fallen by almost two thirds. And residents are happy because the Rippleprint pads have not added a single decibel to the environmental noise generated by the 15,000 vehicles that pass daily.
