It's a beige speaker unit that emits a piercing, high-frequency buzzing sound.
The City of Brotherly Love has been using the Mosquito for the past two years to keep teens away from city parks and recreation centers at night.
At 10 p.m., the devices are activated and a high frequency sound — somewhere between 17.5-18.5 kilohertz — is emitted.
It's main setting is audible to individuals from age 13 to 25 years old, according to the company that makes it, Moving Sound Technologies. (As the cells in our ears age, they typically lose the ability to pick up higher sound frequencies.) However, there have been reports of people outside this age range hearing it, and some audio experts say children could likely hear it, too.
The Mosquito devices have been installed in 31 sites across the city, including at Chalfont Playground in northeast Philadelphia and East Poplar Playground.
"Philadelphia Parks and Recreation began installing the Mosquito devices in collaboration with city council members, partners and the community as part of a holistic effort to deter overnight incidents where they had occurred previously," Lauren Cox, deputy communications director in the Office of the Mayor of Philadelphia, told TODAY.
"Every Parks and Rec site has unique safety needs. These devices are one approach, taken in collaboration with residents and partners, to address overnight safety and vandalism issues. However, due to recent feedback, an internal review of the use of overnight sonic devices is currently underway."
Moving Sound Technologies said that the sound does not produce any harmful or long-term side effects.
"These devices have been banned in some municipalities in Europe," Dr. Hamid Djalilian, professor of otolaryngology and biomedical engineering and director of otology and neurotology at the University of California, Irvine told TODAY.
"The main issue is that children who are young and can't express themselves could be hearing the sound," Djalilian said, citing children with autism as a group who could be affected.
While the devices are meant to keep away those in the 13-25 range, Djalilian said that young children can definitely hear the sound, too. He explained that for most people, "the level of sound would have to be very loud and sustained over a long period of time to cause damage."
Some in the U.S. have filed complaints about the devices, citing age discrimination, including a 28-year-old Washington, D.C., man who could hear the sound at his local metro station (the device has since been removed there). Djalilian said that genetics can play a role in why some people over 25 may still hear the Mosquito, but that it mostly comes down to the natural aging process.
"You could actually determine someone's age by the frequency they can hear sound at. We're all born with the full number of cells in inner ear range 20 hertz to 20,000 hertz — the top of that would be considered dog whistle range. Starting around age 20, we start losing cells in the highest frequency range. So it's age-related hearing loss."
While some with a greater sensitivity to sound may even get migraines with long-term exposure, Djalilian said that the devices are probably safe as long as it's "at a safe level and short-term exposure."