ShotSpotter: Cracking down on deadly Fourth of July celebrations with audio tech

Source: Extreme Tech

 It may sound like an urban legend, but every year people are killed by stray bullets falling from the sky — usually from celebratory shots fired into the air on New Years or the 4th of July. A series of well-publicized deaths has led many states to toughen the penalties for such antics, but until the advent of the clever ShotSpotter system they had few tools for enforcement. ShotSpotter works by deploying an array of listening devices around a city. Loud noises are recorded, analyzed and triangulated by computer, with help from humans. Police can then quickly respond, continuing to get updates on any additional shots from a graphical interface in their patrol cars.

Developed to fight crime

ShotSpotter’s main target is criminal violence. It can report the street address, as well as number of shots, type of weapon used, and even the speed and direction the shooter is moving. All this happens even if no one has called 911 — allowing police to more effectively respond to both reported and unreported incidents. This is more important than you might think, since less than 20% of all gunshots in cities are reported. Even when shots are reported, the caller is often uncertain of exactly where or when they occurred. SST — the venture-backed company that sells ShotSpotter — reports crime decreases of up to 40% in cities where its system is in use.
ShotSpotter is particularly useful on holidays, both because of the increased incidence of gunfire and the use of illegal fireworks. The hundreds of shots of celebratory gunfire are far out numbered by the thousands of incidents of illegal fireworks use.

The technology behind ShotSpotter

SST keeps many of the details of its system proprietary, but some of the basics are known. The ShotSpotter system uses 10 to 12 audio sensors per square mile, with each being able to detect gunshots as far as two miles away. That means over a dozen sensors will have some reading from each gunshot. GPS units built into the sensors are also a critical part of the process. The system relies on the highly accurate GPS-provided clock signal to synchronize the gunshot reports from multiple sensors. From there, triangulation narrows down the possible origin of the shot to a circle about 25 meters in diameter — close enough to indicate a street address in most cases. Noise signatures are compared against a database of known weapons, often allowing the specific model firearm to be identified.
This video provides a glimpse of the ShotSpotter system in action (demo of the user interface starts about 40 seconds in).                

Costing from hundreds of thousands for a small city or town, up to millions for a large city, ShotSpotter adoption is continuing at a rapid rate. It is now being used in dozens of municipalities across the US.