Tokyo's Adachi Ward Office, plans to launch an experiment in May to
see whether a "mosquito" device that emits high-frequency noises aimed
at driving teenagers away can be effective in preventing minors from
loitering in parks late at night.
Amid a rise in vandalism at playgrounds nationwide, many benches and
toilet facilities in Adachi Ward also have suffered damage. The
experiment was decided on because it is deemed that teenagers who
gather in parks late at night caused the damage.
http://www.yomiuri.co.jp/dy/national/20090426TDY03104.htm