Calling the shots

IF A gun fires and nobody reports it, does it make a sound? Some police forces are finding out. On September 3rd the Urban Institute, a think-tank, produced a report based on data from Washington DC’s police “ShotSpotter” system, a network of microphones that covers around a quarter of the city. In the 2011-12 school year, the devices detected 336 incidents of gunfire during the school day. Over half the schools in the covered area had at least one gunshot nearby—most of which went unreported.

Such data are unsettling to parents, but useful to police, who are adopting gunshot-recording technology in ever-greater numbers. DC’s system was one of the first: it has been in operation since 2005. Around 80 forces now use the same technology, which is made by SST, a Californian firm. Microphones are spread out through areas known to have lots of gun crime. Clever software filters out gunshot-like noises such as cars backfiring or men drilling roads; it also triangulates data from several microphones to provide a precise location to officers just minutes after shots are fired.

The data produced are revealing. For example, celebratory gunfire is surprisingly common. In the final six hours of December 31st 2013 ShotSpotter microphones across America apparently picked up 1,186 gunshots—a quarter of the total fired in the whole of that month. In some relatively low-crime areas, the share was far higher. Fireworks are not to blame: they sound different from gunshots. Residents of Miami Gardens, Florida, sometimes shoot bullets in the air to mark victories by the Miami Heat (the local basketball team), it seems.

More usefully, gunshot detectors help cops guess at the extent of unreported gun crime. SST estimates that in the cities covered, guns are fired 396 times for every homicide. Just one illicit shooting in five is reported. Such information spurs changes in how police deal with shootings.

Thanks to location data, they don’t have to spend as much time searching for evidence that a shooting has occurred, such as spent shell casings. The software can even tell whether multiple guns were used, or whether the shooter was moving as he pulled the trigger. Several states now allow microphone analysis to be used in trials. In one case in DC, it helped to clear an off-duty police officer who killed a 14-year-old boy: the microphones suggested that shots had been fired before the officer pulled the trigger.

Some officers say that the data can even reduce distrust between citizens and the police. “We can’t convince people that we care if we don’t respond to gunshots,” says Ron Teachman, the chief of police in South Bend, a struggling industrial city in Indiana. He reckons that last year less than a tenth of gunshots were reported to his force. Now South Bend officers turn up anyway, and can spend time looking for witnesses and reassuring people. In Camden, New Jersey, a strikingly violent place, the recently reformed police force is linking up its system with security cameras, some of which are monitored by concerned citizens.

Putting bucks where the bangs are

Not everyone is convinced. Daniel Webster, a criminologist at Johns Hopkins University in Baltimore, says that many police forces simply do not have the resources to respond to most gunshots. Worse, some police officers complain that the system produces too many false positives—sending officers chasing phantoms—while missing shootings that take place inside buildings or at extremely close range (and so are muffled). In Suffolk County, a suburban district in New York state, police found that just 7% of gunshots identified by their system could be proved to have happened.

Yet the spread of microphones, like other high-tech policing, is likely to continue, says Jim Bueerman of the Police Foundation, a think-tank. Since the 1990s American police forces have been using mapping software and COMPSTAT data to pour officers into the areas where murders and robberies are most common. That approach was often distrusted by older cops, yet helped to turn around the crime wave in places like New York and Los Angeles. Adding data from microphones, cameras and numberplate scanners is a logical next step. The more the cops know about where guns are fired, the easier it may be to silence them.