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Cities Are Using Hidden Webs of Acoustic Sensors to Detect Gunfire

This is what a front line in the data-driven battle to stem urban gun violence sounds like.
July 16, 2014, 10:05pm
Image: Shutterstock

An array of acoustic sensors perch atop roofs and light poles, listening for any disturbances to the ambient rhythms of the urban milieu. It might sound like just another way we're sonifying the city, but this is not an art installation. It's a front line in the data-driven battle to stem gun violence in major cities, and it's rolling tape around the world as law enforcement takes to ShotSpotter and similar gunfire-detection technologies.

The idea is simple: reduce the time it takes police to safely and effectively respond to incidents involving firearms. The clandestine sensor system behind something like ShotSpotter picks up the sound of a shot fired before triangulating the location of that shot using GPS, and then pinging dispatch, who gets a unit headed to the scene, all in about 10 seconds. At least that's the idea.


To hear SST tell it, the advantage to using Shotspotter is that it offers so-called "wide area protection," which is to say it picks up a broader range of sonic concussions than, say, a sensor array that offers only what's known as "point protection," the sort commonly used in SWAT and military applications to pick up far-away rifle fire.

As such, ShotSpotter is ideally suited for "civilian and critical infrastructure[.]" From SST's website:

Unlike counter-sniper sensors which can only measure a limited range of sounds—the supersonic signature of a sniper’s round with a known ballistic coefficient—SST’s wide area protection system measures the full range of impulsive sounds (sounds which are explosive in nature) found in urban weaponry, from sub and supersonic impulses to explosions.

So when the array picks up a shot(s), it tells you where the gun was fired (right down to exact latitude/longitude and street address); how many shots were fired, and at what time; the position of the shooter (and if he's moving, how fast and in what direction); and for later use, "gunfire incident history and pattern analysis," the site adds.

A slew of urban centers, some notorious for high rates of gun violence, are dabbling with this sort of hidden web of intelligent gunshot sensors. Shotspotter has popped up in places like Chicago, San Francisco, Miami, Boston, in Puerto Rico, Rio de Janeiro, and, most recently, in a two-year, $1.5 million trial phase here in New York City, where acoustic sensors will be strategically positioned to listen in on a 15-mile swath of the city. (The city isn't saying exactly which parts of the city will be recorded.)


At least 300 acoustic sensors cover 20 square miles of Washington, DC, where roughly 39,000 separate gunfire incidents have caught the unseen ears of the capital's ShotSpotter system, as the Washington Post reported late last year. ShotSpotter is also now in schools.

At face value, you can see how something like ShotSpotter would be attractive to law enforcement in places like Chicago, NYC, Rio, and beyond, cities that have reached a whatever-it-takes moment in reducing gun-related killings, which disproportionately seem to affect the young and the poor.

But as with the promise of most any other reputed magic techno-bullet, it's just not that easy.

There's the problem of upkeep, for one. Logistically, operating and maintaining this type of city-wide soundsystem requires a lot of attention, to say nothing of funds. As my colleague DJ Pangburn noted, "a city contractor would have to set up all of the equipment and ensure constant connectivity."

Take a city like Birmingham, England, which pulled the plug on its ShotSpotter array when officials reported "technical difficulties," as the BBC notes. West Midlands Police said in August 2012 that only two gunfire incidents, from 1,618 total alerts produced by their web, were ever confirmed, according to the BBC. The trouble didn't stop there:

ShotSpotter had also missed four confirmed shootings. Its conclusion was that resources would be best spent elsewhere. Ch Supt Clive Burgess said the system had "struggled to work" and that in future officers would instead focus on day-to-day community policing, anti-gun education programmes and the work of the counter-gang task force.


There's also the problem of privacy. According to the BBC, certain Birmingham residents voiced concerns about their conversations being inadvertently scooped up by the short-lived acoustic array. Police did their best to assuage those fears, but in the end, the rising threat of surveillance creep was seemingly too much for officials to throw their weight against. After all, something like SpotShotter's technology could without too much effort be patched into the "predictive policing" trend writ large.

"It's not hard to imagine ways to improve a system like ShotSpotter," writes researcher Evgeny Morozov. In his view, rather than detecting gun shots, "new and smart systems" could hone in on those sounds that in the past would've blipped on the acoustic array before a gun was fired. Per Morozov:

This is where the techniques and ideologies of big data make another appearance, promising that a greater, deeper analysis of data about past crimes, combined with sophisticated algorithms, can predict – and prevent – future ones. […] It's the epitome of solutionism; there is hardly a better example of how technology and big data can be put to work to solve the problem of crime by simply eliminating crime altogether.

He's got a point, because the scale is just about there. After all, ShotSpotter arrays have gone live, at one point or another, in over 70 US cities.

But for now, perhaps the biggest thing we have to worry about, before the days of frantic door-to-door, backyard-to-backyard searches by cops looking for suspected gunners are truly behind us, is the underreporting of gunfire among residents who live under hidden webs of microphones.

That's exactly what's happening in Chicago, which turned on its ShotSpotter array in 2012. Over the recent Fourth of July weekend in the Second City, 81 people were hit in 21 separate shootings, as NPR reported. At least 14 people died.