Introduction to How can acoustics technology help police locate gunshots?

October 30, 2006

In parts of cities like Washington, D.C., gunshot-detection systems are improving police response time to shots fired. Gunshots that once waited for responses to 9-1-1 calls from neighbors or the discovery of a body hours or days later now get reported automatically, within seconds of the event.

There are several gunshot-sensor systems on the market. Some use sensors designed to detect the sonic boom of a bullet that travels faster than the speed of sound. Others use sensors that pick up the optical characteristics of a muzzle blast, the explosion that propels the bullet out of the gun barrel. A system recently deployed in Washington, D.C., called ShotSpotter, relies on an acoustics-based, GPS-equipped system that automatically locates the origin of the shot and notifies authorities. A series of acoustic sensors picks up the sound waves of a muzzle blast that radiate outward from the barrel in all directions.

The heart of the system is acoustic triangulation. While the exact technical details of the ShotSpotter system are proprietary, we can get a fairly good idea of how the system works by looking at the process of triangulation.

ShotSpotter uses 10 to 12 sensors spaced evenly throughout each square-mile section of the city it's covering, and each sensor is capable of hearing the sound of gunfire within a 2-mile (3-km) radius.

Because the speed of sound is a known entity -- 340.29 meters per second (0.21 miles per second) at sea level -- the difference in the time it takes for the sound of a gunshot to reach three different sensors can determine the location of that gunshot. Using a built-in GPS system as an accurate time source, three sensors work together to triangulate the location from which a shot was fired. Here's an overview of how a system like this might work:

  1. A shot is fired somewhere in the city. Sensor 1 picks up the sound of the shot. Since each acoustic sensor has a range of about 2 miles, all we know right now is that the shot was fired within a 2-mile radius of Sensor 1.
  1. One second later, a second sensor picks up the sound waves of a gunshot. If sound in this city travels at about 0.21 miles per second, we now know that the shot was fired approximately one-fifth of a mile farther away from Sensor 2 than from Sensor 1. We can draw a circle representing the perception radius of Sensor 2 overlapping the perception radius of Sensor 1 -- since both sensors picked up the sound waves, the shot must have been fired within the overlapping coverage areas. Where the two circles intersect, we have two possible locations for our gunshot.

Circles not drawn to scale

  1. To figure out which of these two points is the location from which the shot was fired, we need to find a third sensor that picked up the sound of the shot. A third sensor, located to the south of Sensors 1 and 2, picked up the sound waves a half-second after Sensor 2 detected them. This would put the origin of the sound about one-tenth of a mile farther from Sensor 3 than from Sensor 2.

Circles not drawn to scale

We now have our gunshot location, at least in terms of distance from the sensors. The system then uses built-in GPS receivers to convert that known point into latitude and longitude coordinates, and passes the information to the nearest system base station via either phone lines (for wired sensors) or radio-frequency signals (for wireless sensors). The base station forwards the coordinates to the nearest 911 call center, which has the equipment to convert those coordinates into addresses and cross streets. The 911 call takers dispatch police to that location. According to ShotSpotter, the system is accurate to 25 meters (82 feet) or less -- far less than the length of a typical city block.

The sensors themselves are about the size of a thick stick of gum. Each one is enclosed in a weatherproof box about half the size of a loaf of bread, which is typically situated on a rooftop or telephone pole. According to ShotSpotter, the sensors are sensitive enough to tell the difference between a gun shot and a car backfiring.

Police officials in D.C. say the system has cut the "shots fired" response time in half. ShotSpotter also records all detected gunfire and corresponding locations for later forensic use. The cost of implementation for the ShotSpotter system can range from hundreds of thousands of dollars for a small area to millions of dollars to cover an entire city the size of D.C.

For more information on ShotSpotter, gunshot-detection systems and related topics, check out the following links: