Note: Areas with no shading may have had gunshots but are not within ShotSpotter’s range. Half of the incidents involve multiple gunshots.

in this square

2009 2010 2011 2012 2013
1 0 0 0 0

121 within a quarter-mile

Can you hear the difference?

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Gunfire

VS

Pyrotechnics

400 feet away

1,600 feet

800 feet

2,100 feet

Each type of sound has a waveform that shows its characteristics, like a fingerprint. Because of this, the system can distinguish a gunshot from fireworks.

The system can be taxed by the sheer number of noises it has to evaluate. It weeds out noises that are obviously not shots and sends about one of every 10 to D.C. police, who make the final determination, often in 2 to 3 seconds.

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A gunfire detection system has captured more than 39,000 gunshots in D.C. since 2006. (The Fold/The Washington Post)

Occasional difficulties

A Washington Post analysis found that distinguishing gunshots from pyrotechnics in the District can be difficult.

ShotSpotter reported gunshots from the source of the Cherry Blossom Festival fireworks and from ballgames at Nationals Park, including three season home openers and two playoff games, where fireworks were used.

About a third of detected gunshot incidents in the city happen on New Year’s Eve or around July 4. Officials explain the high rate as celebratory gunfire.

24%

Fourth of July

10%

New Year's

66%

Non-holiday

Locating the sound

The sound of a gunshot can travel up to two miles, more than enough distance to register on several synchronized sensors. In D.C., an average of eight or nine sensors pick up each shot.

Each sensor records the time it detects a shot. Using the times and distances between pairs of sensors, the system creates curves. The system says the shot was fired where the curves intersect.

Diagram is schematic

Things that can interfere with the system

Alleys

Tall buildings cause sound to bounce around so much that it loses energy and may travel only a few hundred feet. And because sound bends in an arc, a sensor on top of the nearest building may not even pick it up.

Cars

If a person fires into a car and the barrel of a gun is near or within it, the sound is contained and it qualifies as an indoor shot, something the system can’t accurately detect.

Bodies

The sound wave from a shot fired at very close range — within a foot or so — is largely absorbed by the soft tissue of a person’s body and drastically muffled.

SOURCE: ShotSpotter vice president James Beldock, Post analysis of Metropolitan Police Department sensor data. Map powered by Mapbox.