Gunfire gives off a certain sound, a combination of muzzle blast that's recognizable by the human ear and the resulting shock wave from the moving bullet that's more easily detected by technology. Put up enough sensors in your city, and you can filter for the sound of gunfire and even triangulate the source of it.
Police departments have been trying to do this since the 1990s, typically with sensors deployed at intersections that can alert law enforcement of an incident even before phone calls to 911 do.
"But cities can only afford to put that in high-crime areas, nobody can cover the entire city," says Akos Ledeczi, an associate professor of electrical and computer science at Vanderbilt. He has been working for several years on an alternative: an affordable hardware module for an Android smartphone that could do something similar but from mobile positions. "If you put this on squad cars or actual officers," he says, "then you are much better off."
Ledeczi and colleagues at Vanderbilt have been researching the technology with a grant from the Defense Advance Research Project Agency. The most obvious application of their tool is on battlefields, where long-range rifles produce bullets (and shock waves) that travel much farther than gunfire from a handgun can. In urban areas, though, this gunshot detector might be used as a warning system for officers in a close-range gunfight (the sensor could alert headquarters before a radio dispatcher can), or as a system for locating snipers at a crowded, official event.
"Smartphones alone could be used for shot detection," Ledeczi explains, "but typically people carry it in their pocket, and having the microphone on would kill your battery."
These researchers have developed instead a small external sensor that connects to a smartphone via bluetooth. It contains a microphone that does the acoustic processing, and that sensor then transmits data to the phone only when it detects gunfire. The sensor itself can run for about a day without recharging the battery.
Ledeczi doesn't believe there would be much of a commercial market for this hardware among concerned citizens in American cities, but we're not so sure about that. More than 150,000 people listened to the Boston Police Department's scanner in the midst of the marathon bombing suspect's manhunt two weeks ago. What if residents and their smartphones could also listen for evidence ("ears on the street?") that could help law enforcement keep cities safe?