Center Has an Ear to the Ground in Study of Highway Noise Reduction

Those ugly sound barriers that line many of our highways could become a thing of the past if researchers at Purdue University have their way.

The university has just formed the first research center in the country dedicated to the study of the primary source of highway noise--where the rubber meets the road.

Mechanical engineering professor Bob Bernhard, director of the university’s new Institute of Safe, Quiet and Durable Highways at the West Lafayette, Ind., campus, said the underlying philosophy behind the center is that most highway noise comes from the interface between tires and the roadway.

“The background roar of the highway is essentially the tires,” Bernhard said. That noise is frequently punctuated by “single events” such as the roar of a motorcycle or the rumble of a truck engine, but the primary villain is the tires.

The noise is so annoying that states such as California have erected walls along highways to shield nearby homes, sometimes at a cost as high as $1 million a mile.

But Bernhard believes the noise can be brought under control, partly because of a bit of serendipity in Europe. In an effort to improve traction on highways, some European countries, most notably Sweden, began building what they called “porous highways” a few years ago. The surface of the roadway actually has small holes that allow water to drain away.

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But when scientists there began studying performance of the highways, they found something surprising. The roads were a lot quieter.

Bernhard and Vincent Drnevich, head of the school of civil engineering at Purdue, took a look at the European highways, and they think they know why the noise was reduced.

The rumble of passing vehicles is primarily caused by air that is trapped and compressed between the tires and the roadway, the researchers said. The air bursts from beneath the tires, causing “pops and whistles.”

In addition, the tread blocks--the individual pieces of rubber that form the tire tread--and the underlying belts vibrate and radiate energy outward, producing sound like the vibrating cones in stereo speakers.

“We don’t know that for sure, but we think that’s what’s happening,” Bernhard said.

To test their theories, the engineers turned to one of the newest tools in a modern research laboratory--lasers--and are now setting up experiments to measure the vibration of the tire as it turns against a roller.

“The roller has a mirror on it and it shines the little red laser dot onto the tire, and it receives the reflected laser light back and converts that into a vibration measurement,” Bernhard said.

By changing the surface of the roller, and the tread of the tire, the researchers hope to determine which configurations reduce the noise at the tire-roadway interface.

“The research in this institute is really where the rubber meets the road,” said Drnevich, co-director of the institute, which is funded in part by a $3.6-million grant from the Transportation Department.

The surface of a roadway is formed by combining small stones, or aggregate, with a binder, either asphalt or cement. Throughout Scandinavia, and on some highways in France, small holes have been left on the surface.

“You can actually see the texture,” Bernhard said.

The holes apparently keep the air from being compressed as the tires roll over them, thereby reducing the noise. However, one problem has emerged in Europe. The holes fill up with dirt.

To solve that problem, researchers there are experimenting with a multilayer concept where a second layer has even bigger pores, allowing water to carry the dirt down and away from the roadway. Whether that will work remains to be seen.

Bernhard believes an engineering solution can be found, and the lab has a five-year plan aimed at designing an “overlayment” that could be applied to existing roadways.

The lab will also concentrate on tire design, because research has shown that the shape of a tire’s tread can contribute significantly to highway noise.

“Truck tires are particularly noisy, and anything with an ‘aggressive’ tread,” like that on some sport-utility vehicles, contributes to the problem, Bernhard said.

These tires tend to have uniform tread blocks that “tend to sing,” he said. “So not only do you get highway noise, you get a whining tone.”

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That problem became apparent about 15 years ago because of equipment not every Angeleno has: snow tires, which can produce a particularly obnoxious sound. But researchers found that by simply changing the shape of the tread blocks from rectangles to parallelograms, the noise subsided. “All tire manufacturers do that now,” Bernhard said.

Highway noise has become a major economic issue for both auto makers and tire companies, he added. Many vehicles are marketed partly on the basis of the noise level inside the cabin, as evidenced by countless television commercials.

“There is a lot of work being done between the auto manufacturers and the tire companies to pick a tire that [sounds] quiet in the interior of the vehicle,” Bernhard said. “Noise is a quality issue now.”

So if the lab’s research yields quieter tires, Bernhard said, the industry will probably embrace the findings.

And if they can figure out how to build “porous” highways that don’t clog up, some of those sound barriers might not be necessary.

Then maybe someone can figure out how to get mufflers on all those motorcycles, and quiet brakes on all those screeching buses, and truck engines that just purr. Hey, at least it’s a start.

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Lee Dye can be reached at leedye@ptialaska.net.