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Making noise about tire sound levels

Here's something to squeal about: Though most tires are designed to be quiet, some are made to be loud, raising public nuisance issues.

May 12, 2004|Ralph Vartabedian | Times Staff Writer

A good set of tires has always made a big difference in safety, fuel economy and handling, but one area that is getting new attention is the quality of tires to reduce or increase the amount of noise a vehicle makes.

Tire manufacturers are racing to develop tires that reduce noise, aiming to meet tough new standards mandated by European and Asian nations that want to reduce noise pollution. Though U.S. regulators are not taking the same aggressive approach, motorists here will still benefit from this research, according to tire industry experts.

A number of readers have asked recently about vehicle noise and how to reduce it, both the noise that occurs inside a car and the kind that affects the community.

In general, the trend in the automobile industry is to engineer quieter components such as tires, and by itself that effort would be producing quieter cars, according to tire makers. But a segment of drivers are pushing in the opposite direction and some actually intentionally want to make noise.

When it comes to tires, community noise laws generally do not regulate tire noise, unlike other vehicle noise sources such as exhaust or stereo systems. As a result, some vehicles equipped for off-road travel or stylized to look like off-road vehicles can produce noise volumes dramatically above the average car or even light truck.

"You can identify a class of consumer who wants a noisy tire and who thinks it gives good off-road traction," said Mike Wischhusen, director of industry standards and government regulations at Michelin.

But Wischhusen said that noisy tires do not necessarily produce better traction and that not all knobby tires have to be noisy by design. He added, "Consumers who intend to make noise are not our targeted market."

The market, however, is served by off brands, typically imported, with poorly engineered treads and atrocious acoustics. The difference in sound between a quiet tire on a car versus a noisy tire on a pickup truck can easily be 10 decibels, or roughly double the noise -- and that does not include the increased engine and exhaust noise of the bigger vehicle. It is one reason why noise is becoming a factor even in residential neighborhoods without high traffic volumes.

Apart from the consumers who intentionally want noisy tires, many others are seeking larger and wider tires for big vehicles such as sport utility vehicles. The aggressive look of such tires appeals to a lot of consumers, even though they never take the vehicles off road or even onto rugged back roads.

So, the yin and yang of tire noise is the industry push for quiet tires and the sometimes opposite push by consumers. Clearly, the majority of consumers want quiet tires and the bulk of research is aimed at improving tread design to decrease noise.

Tire acoustics are a complex science that is getting significant attention in tire research labs, according to Karl Fundkvist, a design engineer at Goodyear who specializes in acoustics.

At low speeds, the engine and exhaust system are the primary sources of noise, but at about 35 mph tire noise comes into play. At freeway speeds, the tires account for about 60% of the noise a typical car makes, Fundkvist said.

Aggressive tread patterns do not necessarily have to be loud, Fundkvist said, though without careful engineering they can produce a signature thunking sound that can be heard for hundreds of feet ahead and even behind a pickup traveling 35 mph.

A knobby tire generally makes more noise than a tire with small tread elements, because the knobby tire is less round. If the knobs are large enough, each element slaps the pavement as it rotates, engineers say. A well-designed knobby tire can have treads that are engineered to reduce the noise, however.

In selecting a quiet tire, it's helpful to understand just how a tire makes noise.

As a tire rotates, it traps tiny air pockets between the treads and the pavement. The grooves get squeezed under the weight of the vehicle, compressing the air. As the tire rotates, the pressurized air pops out and produces a vibration or sound.

Tires also make noise because the tread elements squirm under the weight of the car, slipping on the pavement. You can hear this effect in parking garages with highly polished concrete floors.

Thus, a bald tire might be fairly quiet, but not very safe for stopping or handling. So, safety, acoustics and economy involve some trade-offs.

Engineers have found that they can package a safe, economical and quiet tire by carefully designing a tire's tread. Typically, tread elements are staggered so that different blocks of tread hit the pavement milliseconds apart from each other. The leading edges of tread elements have minute angles to further spread out the noise.

Engineers have also found that carefully designed treads produce sound across a wider frequency, which is less harsh sounding than a narrow frequency. Think of a speaker with a resonant pleasant voice, versus a person with a hard shrill one.

Ironically, the tires equipped on new cars are generally quieter than the replacement tires purchased by motorists, said Wischhusen, the Michelin expert.

"Consumers are more worried about wear and value, not noise," he said.

Ralph Vartabedian can be reached at ralph.vartabedian@latimes.com.

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