It's called "Shotspotter", and it allows the police to zero in with pinpoint accuracy on locations where gunfire has been detected.
A 2006 study of test shots fired at the Charleston Navy Yard, conducted at the company’s request and financed by the National Institute of Justice, found that ShotSpotter correctly detected 99.6 percent of 234 gunshots at 23 firing locations. The system also located 90.9 percent of the shots to within 40 feet.
Sam Sutter, the district attorney in Bristol County, Mass., called ShotSpotter “an extremely valuable tool” that had helped his office bring charges in four nonfatal shootings.
“In my view legally,” he said, “what is said and picked up by the ShotSpotter recording does not have the expectation of privacy because it’s said out in public, and so I think that will turn out to be admissible evidence.”
James G. Beldock, a vice president at ShotSpotter, said that the system was not intended to record anything except gunshots and that cases like New Bedford’s were extremely rare. “There are people who perceive that these sensors are triggered by conversations, but that is just patently not true,” he said. “They don’t turn on unless they hear a gunshot.”