In 2007, the District assumed ownership of the detection system, expanding its coverage in the years that followed. ShotSpotter now reaches into six of the seven police districts and covers about one-third of the city. Its greatest coverage is in Southeast and Northeast, records show.
ShotSpotter is also linked to a system of closed-circuit cameras, which police hope will capture the aftermath of shootings in real time. To guard against vandalism, officials do not publicize the sensors’ appearance or reveal their locations.
Also explained is one of the more useful distinguishing properties of gunshot noise:
The blast of a gun is different from other explosive sounds because it is directional, meaning that the noise changes its frequency as the bullet moves through space. A person may hear a gunshot a half-mile away if the gun is fired toward him. But a person 200 yards away may hear nothing if the gun is fired away from him.
It’s worth a read.