PHILADELPHIA Ask any of the teenagers hanging out at the Chalfont recreation facility in Northeast Philadelphia, and they can tell you what the Mosquito sounds like.

"Nails on a chalkboard."

"A dog whistle."

"Like a screeching sound that echoes."

Usually, only the young, ages 13 to 25, are able to hear the high-frequency noise emitted by the Mosquito, a machine named after the sound it produces.

And that's exactly the point. The machine is meant to prevent youth vandalism and loitering at city recreation centers after hours. The small white boxes have been installed at four recreation facilities in the Northeast, and the city hopes to have the Mosquito and security cameras at every one of the its 154 recreation centers by 2017.

The Chalfont facility, at 4330 Deerpath Lane, was remodeled in 2012, and with that came four Mosquitos and security cameras. It was the first center to use the machine.

"Under the overhang we have here used to be a nice, big drinking spot. With kids hanging around, the graffiti and all that would happen," said Chalfont recreation leader Christine Reilly.

"We have seen a dramatic difference," she said, since the Mosquitos and security cameras were installed. There are no longer graffiti on the walls.

The contraption is also used at the Boyle, Junod, and Mayfair Recreation Facilities, said deputy recreation and programs commissioner Susan Slawson. The machines are set on a timer, and come on daily from 10 p.m. to 6 a.m.

At Boyle, on Stevens Road in Somerton, the difference has been less noticeable.

"To begin with, crime has always been at a minimum, maybe a few isolated incidents," said recreation leader George Geiss. "Most people who frequent here probably don't even know they're there."

The initiative to put cameras and Mosquitos in all recreation facilities will cost the city about $4.6 million, according to Councilwoman Cindy Bass' office.

"We decided to install cameras and the extra protection of the Mosquito," said Bass, chair of Council's Committee on Parks, Recreation, and Cultural Affairs. "People are really excited to see this level of investment."

Michael Gibson, president of Moving Sound Technologies, the Vancouver-based company that sells the new technology, said the Mosquito has been installed in several U.S. cities and in all the major cities in Canada.

The frequency and decibel rate the machine emits is "far below" the rate that might damage human ears, he said. Research has shown, the company website says, that most people older than 25 have lost the ability to hear at this frequency range.

It's more annoying than anything, adolescents say. They can hear it even when they are passing through.

Daniel Spotts, 26, hears the high-pitched buzz when he drives by the park at night. He doesn't think the Mosquito is worth it.

"The cameras have made more of a difference," he said.

Dan Milillo, 17, lives with his family across the street from the recreation center. He doesn't mind it too much.

"I don't hear it inside the house," he said. "When I stand outside the house, it's only a very faint noise."