Just read about an ingenious new technology “ShotSpotter“, imagine a person firing a handgun and automatically within minutes police arrive without anyone calling them..
How it works? Simple, a number of microphones are placed on top of buildings in larger cities, they detect the special sound pattern of gun shots and by triangulation they can determine the vicinity where the shooting took place.. Clever..
PDF presentation;
http://www.shotspotter.com/news/articles/2007/4%20-%20April/Wired%20Magazine/Wired%20Article%20Eprint%204.10.07.pdf