Shot Spotter

"In the past, the best information the police could hope for was a neighbor calling to say, ‘Sorry to bother you, but there may have been a shooting somewhere in my neighborhood,'" says ShotSpotter CEO James Beldock. "Our system can immediately tell them that, say, 11 rounds were fired from a car going 9 miles an hour, northbound, in front of a specific address on Main Street. In some situations, ShotSpotter could get someone on the scene within a minute. That's a level of situational knowledge police have never had."

This kind of coverage requires an array of 12 to 20 specialized sensors per square mile. Roughly the size of a medium pizza and designed to look like a rooftop fan, each sensor contains up to four small microphones. If one of these units detects a loud noise, it forwards a recording to a server at police headquarters along with three pieces of information: location, time, and general direction the sound came from. If a sound is detected by only one sensor, it's probably too quiet to be gunfire, and in any case, the system needs data from three sensors to pinpoint the location of a noise. If several sensors report an event at the same time, the server gets to work. First, the software performs an analysis to categorize the noise as gunfire, firecrackers, bottle rockets, helicopters, or other. If it determines the event was a gunshot, the program makes a simple calculation to triangulate the sound's origin to within 80 feet or less.

Experienced ShotSpotter users can tell a lot about an incident by listening to the recordings. Some say they can distinguish between the pop of a handgun, the crack of a rifle, and the emphatic blast of a shotgun. The company plans to update the software to recognize these variations automatically.