Michael Reyes strolled onto Coney Island's beach in Brooklyn one sultry afternoon this week proudly bearing his "boom box," a nearly yard-long radio with 50-watt speakers.

Placing the 35-pound stereo carefully on the rim of a trash can, Reyes and his friends turned it on full blast. The can rattled, and nearby sun-worshipers rolled their eyes in dismay.

Reyes, a 22-year-old security guard, had to turn down the sound to hear a question -- and his answer was no, it didn't really bother him that he might be disturbing other beachgoers.

"There's plenty of room here for everyone to make noise," he said, shrugging at the empty stretches of sand around him.

But for those who cannot bear boom boxes at any decibel level, a silent sliver of serenity has arrived in New York: "radio-free zones." Those who want silence need only move their towels and suntan oil to one of several designated areas on four of the city's beaches, or limit their Central Park escapades to the Sheep Meadow in the southwestern corner of the park.

Nearly a half-mile of New York City's 14 miles of beaches and 15 of Central Park's 840 acres are affected by the ban. People playing radios in those areas without earphones are subject to a $50 fine and may have their blasters impounded by city police or Parks Enforcement Patrol officers.

"We are taking this step because people go to parks and beaches to escape the noise and confusion of city streets," Mayor Edward I. Koch said at a Central Park news conference last week. "The law doesn't restrict your right to have a radio; it simply restricts your right to play a radio in areas where it might annoy other people."

Parks and Recreation Commissioner Henry J. Stern said the ban was triggered by "hundreds of complaints" in recent months about noise at the city's parks and beaches.

"High technology has made radios much larger and louder than they have been in the past," Stern said. "The little box that used to entertain a family now entertains an acre. People just don't want to be subjected to other people's noise anymore."

So far, according to Adrian Benepe, a spokesman for the Parks Department, six summonses and nearly 50 warnings have been issued.

These so-called radio-free zones were established on Memorial Day, as an experiment, at three small stretches of Orchard Beach in the Bronx. The results so pleased Koch and Parks Commissioner Stern that they decided to expand the ban to parts of Coney Island in Brooklyn, Rockaway Beach in Queens and South Beach on Staten Island, and to the Sheep Meadow.

Nearby, on Long Island at Jones Beach State Park, three quiet zones have been quiet since Memorial Day.

"It's nice to be on the beach without all that constant noise," said Robert Khaleel, 32, a voluntary youth worker who accompanied 20 teen-agers to Jones Beach last weekend.

His cousin, Tara Khaleel, 27, an employe of Chemical Bank in Manhattan, agreed: "The mayor should make the whole city a radio-free zone."

On Coney Island, a half-mile away from the amusement park, Enzo Villela, 59, savored the silence of the quiet zone designated by signs with a red slash over a big box radio.

"I think it's a good idea. Some of the young kids simply play their music too loudly," he said.

In the shadow of the giant Ferris wheel, where the boxes still boom, strains of rock and roll drown out the crashing of the waves. Here, it's thumbs down on Koch's move.

"We have a right to play our music if we want to," said Nelida Martinez, 15, bouncing to the beat of Madonna's "Material Girl."

"I think it stinks," said Marian Demenico, huddled around a small portable radio with four friends. "He has more important things to worry about than the radios on the beaches."

Demenico's friends, women in their mid-thirties, nodded.

"I can't wear any jewelry here. I'm afraid of, you know, chain snatchers," said Dolores Vitella. "I'm sitting here worried that someone will steal my hubcabs. The beaches are dirty, and there aren't any restrooms. And Koch worries about radios? Give me a break."