"Treading Lightly," Feature Article, April 2003

treading lightly

Paved ribbons of speed make for noisy neighbors.

by Paul Sharke, Associate Editor

Car tires rolling over a couple of hundred meters of corrugated roadway in Villepinte, France, play 12 seconds of music stereophonically as the vehicle passes by at 50 km/h. Dedicated in 2000, the musical road has already drawn enough complaints from locals to force a partial paving over. But Ulf Sandberg, author of the Tyre/Road Noise Reference Book (Informex, Kisa, Sweden, 2002) and adjunct professor of tire and road noise at Chalmers University of Technology in Goteborg, Sweden, said that, while visiting the site in 2002, he could still make out the patterns and hear the sounds of the 28-note melody.

Perhaps the tune, composed by Gaellic Guillerm, grew wearying upon repeated hearings by area residents. People living alongside busy highways would probably empathize with the inhabitants of the Paris suburb. At least the steady whine of tires on highways doesn't intentionally try to draw attention to itself.

Drivers approaching the musical road near Paris are instructed to stay between solid lines to hear their tires play a melody. The tone deaf can follow a dotted route to play only road and tire noise.

Still, a regular input of highway noise quickly grows annoying. The coupling of transportation noise and annoyance levels is something that the Shultz curve, developed in the 1970s, tried to do, said Gregg Fleming, chief of environmental measuring and modeling at the U.S. Department of Transportation's Volpe Center in Cambridge, Mass.

The problem isn't new. The Federal Highway Administration Noise Abatement Criteria require that countermeasures be considered when highway noise in residential areas exceeds 67 decibels. For years, highway builders have attempted to squelch road noise by erecting sound barriers. While they're able to block traffic noise for residences situated adjacent to a highway, the barriers do little in attenuating the long-range buzz of a busy highway that can travel far, according to Bob Bernhard. This, coupled with a cost that can run $2 million a mile or more, has led researchers such as Sandberg in Europe and Bernhard, who directs Purdue University's Institute of Safe, Quiet, and Durable Highways in West Lafayette, Ind., to seek solutions nearer to the spot where rubber meets road.

Noise Maker

"The generation mechanisms are extremely complicated," Sandberg said of the phenomenon of tire noise. The tread is the primary contributor to the making of noise, he said. Sidewall design can change the level of the sound that radiates away from the tire, but the tread pattern itself remains the predominant source of the sound.

Think of individual tread elements as blocks that hammer the pavement as they move into, traverse, and exit the tire's contact patch. That hammering sets up vibration in the blocks. The pavement's texture also plays an important role in the kinds of vibrations that develop in the tread and sidewall. Pavement texture can excite these vibrations. Vibration from the blocks accounts for low- to mid-frequency sound.

The movement of air through and around the tire takes responsibility for many of the mid- to high-frequency sounds generated as tires roll across pavement. Air in the grooves can create an "organ pipe resonance," Sandberg said. Air in other cavities can produce a Helmholtz resonance, the same effect that's created by blowing across the open neck of a bottle. The displacement of air at the leading and trailing edges of the contact patch makes noise, too.

"Stick-snap" generates high-frequency noise. As tire rubber presses down on the road, researchers think it adheres slightly. The tire must pull the block away from the pavement as it exits the contact patch, Sandberg explained. Hence, the "snap." A similar phenomenon is believed to occur tangentially to tire rotation. Researchers dub that phenomenon "stick-slip."

For every study looking into tires as a source of noise, there are at least as many, and probably more, investigating roads as attenuators and dampeners of noise, experts say.

Some tire manufacturers claim very little noise difference from one tire design to another, then "prove" the claim by testing only three or five tires, Sandberg said. If they were to increase that sample size and include the variations that affect tires and pavements, they might find differences in tire noise akin to differences in noise found among various pavements, he said.

A recent feasibility study conducted by the Purdue team examined four tires and three pavements. Tires were differentiated by the construction or makeup of the carcassthe area of the tire that underlies the tread (specifically, cap ply, no cap ply, and filler heights of 0.35 or 1.75 inches). Tread pattern, tire pressure, rubber compound properties, and so on remained identical from tire to tire. The four tires ran at three speeds around a laboratory drum that was surfaced with smooth concrete, textured concrete, and porous concrete pavements.

According to Bernhard, very little difference in tire noise arose due to variations in construction of the tested tires. But as the pavements changed, a distinct shift in the spectral signature appeared that carried through from tire to tire.

The 12-foot drum at Purdue's Institute of Safe, Quiet, and Durable Highways holds various surfaces over which two tires roll at 30 mph under a weight of 1,000 lbs. each.

Typical A-weighted spectra of tires running on the smooth and textured concrete showed highest amplitudes around 1,000 Hz with fairly gradual rises and falls. On porous pavement, however, the sharp spectral peak typically occurred near 800 Hz, then dropped well below the amplitude of the curves for smooth and textured concrete above 1,000 Hz.

Although the porous concrete had the highest overall sound pressure level due to the 800 Hz spectral peak, its behavior above 1,000 Hz indicates some of the normal sound-generating mechanisms are being reduced significantly, Bernhard said.

Sandberg called the peak that shows up between 700 and 1,300 Hz "remarkable" because "it totally dominates the sound emission" when the sound is weighted by an A-weighted filterone that matches the filtering effect of human hearing.

After 30 years spent studying tire noise, Sandberg has begun seeing the peaks increase in recent years, something he attributes to "a number of effects and design parameters [that] happen to coincide unfavorably in this frequency range." This, despite efforts by manufacturers years back to avoid "tonal" components in the noise by "randomizing" tread patterns, is leading to the construction of some tires that produce sounds which may be "extra-objectionable," he said.

Arizona Highways

Although the United States lags behind Europe and Japan in the pursuit of quieter roads and tires, Arizona and California are probably ahead of the other states in this area. That's due, in part, to Arizona's continued work with rubberized asphalt long after other states had given up on using it. Congress mandated the practice, which uses recycled tires in pavement, some 20 years ago.

In the ensuing years, Arizona may have experienced fewer durability issues with rubberized asphalt, partly because of a climate that is kind to the material. Other states ran into durability difficulties with rubberized asphalt and switched back to more conventional surfaces.

Durability and noise are but two elements of the highway equation, which also includes life-cycle considerations, safety, and maintenance, Bernhard said.

"I didn't know highways could be made quieter," is the way many people react when they come upon such roadways, Bernhard said. "There is a large pent-up demand for quiet highways from people who have been told they have to live with the problem," he said.

Even as most states were struggling to balance their budgets, public demand for quieter highways in Arizona still managed to push through funding late last year for the application of rubberized pavement on 100-plus miles of roads, to be finished in 2005. Residents like what they are, or aren't, hearing, it seems.

The rubberized asphalt that Arizona's transportation department plans to use is a crumb rubber product recycled from tires. The tire granules are added to a liquid asphalt and stone mixture just before being applied as a 1-inch surface coat on top of 12 to 14 inches of concrete. Arizona DOT claims a 3- to 5-dB reduction in noise for its rubberized asphalt, which can be applied around the Phoenix area only during the region's mild springs and falls.

A 3-dB reduction is considered just noticeable, Bernhard said, while slashing 10 dB about halves the perceived sound level. A 10-dB reduction for homes nearby is the typical target for a row of barriers. Anecdotal evidence indicates that the perceived noise reduction of quiet highways may be greater than conventional measurements can predict.

In Europe, practically all the northern countries are looking at pavements as noise sources and doing what they can to encourage innovation in sound-deadening technology. There, as well as in Japan, 5- to 7-dB reductions have been accomplished using double-layer porous asphalt instead of dense asphalt or concrete surfaces, Sandberg said. But as the pores clog over time, noise rises. Periodic cleaning with water jets may help to restore some sound-absorbing qualities, he said.

The surface is used with some regularity on roads in the Netherlands, while other European countries and Japan are continuing to experiment with it on a trial basis.

Initially, at least, this kind of road surface could produce a 7- to 10-dB reduction in noise on U.S. highways, where transversely tined cement concrete is common. Such transversely tined material is generally noisier than the pavements common in Europe and Japan, Sandberg said.

California, meanwhile, is close to finishing a five-year study along a section of I-80 that has been paved with open graded asphalt, according to Paul Donavan, a senior consultant with Illingworth & Rodkin of Petaluma, Calif. Even after a prolonged period of traffic use, the road surface is maintaining a consistent 5- to 6-dB reduction in noise levels, he said.

His firm has been studying a variety of concrete and asphalt surfaces around the state and has been able to verify what Europe and Japan have been finding, that about 10 dB separates the noisiest concrete from the quietest asphalt.

But what asphalt gains over concrete in noise reduction it gives up when it comes to durability. Concrete costs more to place. So, asphalt and concrete proponents continue arguing over life-cycle costs, Sandberg said.

As a result, the federal government does not yet recognize quiet pavement as a means of sound abatement, Donavan said.

Made to Measure

A laboratory drum is one of three main methods of gathering tire noise data. The other two, coast-by and close proximity, bring the measuring instruments outdoors.

The coast-by method most closely resembles what a person standing beside a highway would hear as traffic roars by. To isolate tire noise from engine sounds, the vehicle glides unpowered past the measuring microphone. Several runs are usually required in order to average out the background noise of other traffic. The close-proximity method mounts a tire on a trailer and places a gang of microphones near the contact patch. In many instances, a shroud covers the tire to isolate it from external sounds.

A typical trailer arrangement hiding an enclosed tire for making close-proximity measurements of noise. This approach opens a multitude of road surfaces to potential study.

The indoor drum method rolls a tire over a curved band of road surface, and is probably the quickest method for amassing a batch of tire noise data.

Doug Hanson, an assistant director of the National Center for Asphalt Technology at Alabama's Auburn University, said the center maintains a test track with 46 different asphalt surfaces. It's currently evaluating the noise characteristics of seven different auto tires at speeds up to 45 mph. The center is also conducting road tests in Alabama at 60 mph. A close-proximity study for the Michigan DOT found Portland cement concrete to be the noisiest road surface, followed by dense graded asphalt, stone matrix asphalt, and open graded asphalt mix. An aggressive tread was noisier than a more passive design in every case, usually by about 1 or 2 dB. About 3 dB separated the noisiest and quietest pavements for both tread types.

Measured from a few inches out, a tire can sound as loud as a chainsaw. The average driver, ensconced in the soft, cushiony interior of a modern automobile, rides close to but completely unaware of the buzz saw sounds coming from beneath the carriage. Highway noise outside the car might not be much of a concern in rural states. It may be a bigger deal in Europe due to the closeness of people and roads there. But for urban areas in the United States, where great bunches of homes are clustering closer and closer to heavily traveled arteries, the unmuzzled open road may be nearer than ever to singing its swan song.