Machine Harnesses Sound Science To Probe Causes Of Road Noise

"This work will eventually lead to a more precise understanding of the causes of highway noise," said Robert Bernhard, co-director of Purdue's Institute for Safe, Quiet and Durable Highways. "The ultimate goal is to create quieter pavements and tires because highway noise is a major environmental irritant."

A 38,000-pound, 12-foot-diameter circular machine designed at Purdue makes it possible to test numerous types of pavement surfaces and compositions in combination with various tire designs. Curved test-pavement sections fit together to form a circle, and two tires are rolled over the surfaces at varying speeds while microphones and other sensors record noise and data.

Because no other equipment in the United States can test any combination of pavements and tires, researchers expect their new Tire/Pavement Test Apparatus to yield a wealth of data, said Bernhard, a professor of mechanical engineering and director of the Ray W. Herrick Laboratories at Purdue.

Unlike most other test equipment, the Purdue machine is designed so that pavement test samples remain stationary while tires roll over the surfaces. Keeping the pavements stationary is critical to accurately mimic actual road conditions. Many other machines use designs in which a stationary tire rides on top of motorized steel rollers.

The rollers cannot be made of actual pavements, however, because the spinning motion would cause the pavements to disintegrate. The steel rollers are covered with various surfaces, but those surfaces do not possess the exact characteristics of real pavements, and it is difficult to change the surfaces for new experiments.

Researchers also measure highway noise by towing a tire behind a vehicle. That method, however, is limited to the type of pavement that is available on existing roads or test tracks, and the test results are affected by uncontrollable environmental conditions, such as other traffic, wind noise and changing weather. The Purdue engineers are able to analyze numerous types of pavements by changing test samples, and the experiments can be carried out inside a laboratory where acoustics, temperature and other environmental conditions can be carefully controlled.

"The thing we can do that nobody else can do is test a large range of pavements and tires in a way that promises to isolate the precise causes of highway noise," Bernhard said.

Preliminary findings were discussed for the first time on March 25 during the 89th Purdue Road School, an annual gathering for government officials, traffic experts and engineers to discuss a range of transportation issues. A report on the first findings is available online at http://widget.ecn.purdue.edu/~sqdh/publications.html.

Engineers have initially used the machine to test three types of concrete surfaces - smooth, textured and porous - and four tire designs, which had varying degrees of stiffness in their sidewalls and tread bands, or the steel belts that encircle the tire underneath the treads.

An array of five microphones recorded the noise levels at various distances and at several frequencies, or tones, emanating from the interface of tire and pavement.

"There were pretty significant differences between the three pavements, but we saw pretty much no difference in the sound radiation from the tires," Bernhard said. "All four tires were pretty much the same, which means the sidewalls and the tread bands we tested really didn't have much of an effect on the tire noise. However, other tire characteristics, such as the rubber compounds used, may have more effect in controlling noise."

Preliminary information showed that the porous pavement generated the least noise.

"What we think is happening is that certain elements of the pavement and tire amplify sound, but we really don't understand this completely yet," Bernhard said.

More work is needed to discover precisely how the noise is generated. The researchers will soon begin testing tires and pavements that contain embedded sensors, which will provide more data about noise generation.

"We are going to go after what's happening at the interface of the tire and road surface," Bernhard said.

Engineers have several theories for the root causes of highway noise. One theory is that as a tire's tread blocks strike pavement, the rubber blocks vibrate, radiating noise.

"We want to find out whether a tread does in fact vibrate, and if so, at what frequency it is vibrating," Bernhard said.

Another theory is that grooves between tread blocks act as tiny organ pipes as the tire spins, producing noise. Still another theory is that the tread blocks stick momentarily after striking the pavement, making a noise as they pull away from the surface.

The Purdue machine, which cost about $250,000, was paid for with a combination of funding, including money from the U.S. Department of Transportation, contracts with several tire companies and a gift from Ford Motor Co.