Cities from New York to Denver are giving motorcyclists the silent treatment.

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And that worries riders rights groups, which fear that a wave of ordinances aimed at muffling Harley-Davidsons, hushing Hondas and stifling Suzukis will create a confusing patchwork of laws that motorcyclists won't be able to navigate. The motorcycle industry is concerned it could turn these frustrated riders away.

"From our perspective, this creates enormous problems for us because people notice the one motorcycle that makes a lot of noise," said Bill Wood, spokesman for the American Motorcyclist Association. "They don't notice the 50 that pass that don't. So there's a perception that motorcycles are noisy."

Ordinances come in many forms. Some are against certain types of products — like mufflers that would rattle the apples off of trees — while others are aimed more on the intent of the driver, who may want to turn some heads or rile up the neighbors on a Sunday afternoon.

—As of July 1, riders in New York City are subject to a minimum $440 fine for having a muffler or exhaust system that can be heard within 200 feet.

—In Lancaster, Pa., starting this month riders — and all motor vehicle drivers — could be ticketed for drawing attention to themselves, whether by creating too much noise by revving their engines or doing hard accelerations. Tickets start at $150.

—As of July 1, motorcyclists in Denver could be ticketed $500 for putting mufflers on their bikes made by someone other than the original manufacturer, if the bike is 25 years old or less. These so-called after-market products can be louder than their manufacturer-made counterparts.

Denver's plan is unique because it targets the after-market equipment. Wood said it limits riders' freedom to choose what products to use. Many motorcyclists who need to replace parts use these products, rather than go to a dealer, which can be more expensive, Wood said.

Ordinances restricting motorcycle noise have been around for years. The American Motorcyclist Association does not track the numbers of such ordinances and often only hears about them just as they're being passed, Wood said.

The association would rather see an ordinance that targets all vehicles or uses a decibel test to measure actual noise output.

The changes leave riders confused, said Pamela Amette, vice president of the Motorcycle Industry Council, the industry's trade group. Enforcement can be subjective, too.

The Council is working with the American Society of Engineers to establish a sound test that would help equalize enforcement.

A similar test has been set for off-road bikes, and several states have adopted it, Amette said.

The group hopes to have the test ready next year. The new tests could even heighten demand for quieter systems, she said, because riders will know what they need.

"Unless it's very precise and adopted uniformly, then it's just really not fair to the riders and to the industry," Amette said.

The stakes for the industry are big. There were 1.1 million new motorcycles sold for $9.8 billion in 2005, the most recent year available, the Council said. Parts, including those after-market mufflers, accessories and riding apparel, were an additional $2.8 billion.

Noise complaints of all types are on the rise, as more Americans feel they are losing control of their neighborhoods, said Ted Rueter, who leads a national anti-noise group. Denver's ordinance is music to his ears.

"I think more and more people are putting pressure on communities," said Rueter, director of Noise Free America, based in Madison, Wis. "That fact that Denver has done so is going to give a lot of encouragement to people who love peace and quiet."

Harley-Davidson Inc., which tried in the 1990s to trademark its products' distinctive rumble, is monitoring the growth of anti-noise ordinances that target motorcyclists, said Rebecca Bortner, a Harley spokeswoman.

The Milwaukee-based motorcycle maker feels the issue is less about the equipment and more about what riders do with it. The company asked its dealers a few years ago to stop carrying the loudest of after-market mufflers, straight unmuffled pipes, Bortner said.

Harley is asking dealers to encourage riders to be considerate, she said. Sometimes, that means riders should put a bike in neutral when they're in a driveway. They can fire up the engine down the street.

"You hear about jackhammers in New York City and people who live close to airports," Bortner said. "We are very sympathetic to that, but our stance is that we're really encouraging riders to take steps to be considerate and socially responsible about it."

All motorcycles sold for road use in the U.S. are subject to federal noise laws keeping them within a certain range of decibels, below 80 decibels from 50 feet away, said the industry council's Amette. A good rule of thumb is that your average motorcycle — as approved by government standards — should hum like a sewing machine, she said.

But some bikes are louder. That happens when bikers buy after-market equipment, either for the sound or for more heightened performance.

Manufacturers divide their motorcycle products into two types — for highway use and off-road, which is more performance-based and sometimes louder. You can buy a road bike and then add an off-road muffler, which means it'll be louder on the street. That's what Denver's ordinance aims to stop.

Enforcement has been minimal, said Wade Eldridge, a motorcycle-riding lawyer representing a handful of riders who have been ticketed so far. He said they're trying to get the ordinance declared unconstitutional because it creates two classes of motorcyclists.

Rider Dave Christy, of Golden, Colo., said he knows some bikers now avoid Denver rather than deal with the new ordinance. The problem is riders who enjoy being loud without a thought to people who live nearby, he said. Christy, a 53-year-old mechanic, uses after-market mufflers on his bikes, but he knows when not to be loud.

"What came out of the ordinance is a result of what motorcyclists pretty much brought down on themselves," he said.