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Roll Call

ShotSpotter: Acoustic Surveillance Technology

Gunfire Detection

By Aric DiLalla

ShotSpotter A gunshot goes off downtown. Within 30 seconds, the police are notified and on their way. No one has called 911 nor has an officer witnessed the shooting. Instead, high above street level, a sensor within a box has charted the noise. In rapid succession, it fires off alerts to law enforcement and a command center. This is the new reality with ShotSpotter, and its making cities safer.

ShotSpotter is a Jameco customer so we asked if we could learn more. For nearly a decade, the company has been working to perfect the system that tracks unusual noises and sends out alerts if danger is detected. Acoustic sensors are placed on rooftops and telephone poles and are triggered by loud ambient noise. If a gun fires, various acoustic sensors in the area will triangulate the location and alert a 24-hour command center. The sensors, which are wireless, communicate with the base on the AT&T network, with which ShotSpotter has a partnership. The alerts are also dispatched to nearby police stations.

The experts in the command center will receive both a pinpointed location and an audio snippet of the noise. If the gunfire analysts in the command center determine the sound is gunfire rather than a car backfiring, they push an alert through to local law enforcement. The command center is charged with making sure law enforcement doesn't waste its time on false alarms, but they're also able to provide additional information like the number of bullets fired - something that can help indicate what type of gun is being used and whether the shooter is on foot or in a car, based on how much the location changes.

"Not only are you getting the alert," Director of Marketing Sherry Prescott said, "but you're getting some intelligence from the analysts."

Law enforcement, which is provided with data on a map so that they can head toward the root of the issue, normally receives all of the information within 30 seconds of the bullet being fired.

Douglas McFarlin, ShotSpotter's vice president of engineering, said creating this chain of information was anything but easy. And that begins with the device itself.

The sensor, as McFarlin explains it, is like a "mobile device without a user interface." It uses a custom-designed single board with an embedded cellular module. The device uses the cellular network to send back the data including an audio section that brings in analog signals from a microphone and a GPS module that provides accurate location and timing. The device also has a micro-processing section that runs Linux and a firmware application.

This is all powered by tapping into an AC main with a power supply (Jameco provides ShotSpotter with these power supplies). The sensors then tap into available sources for low voltage power on rooftops or telephone poles.

The challenges of keeping a device this complex in working order around the clock are anything but simple. For one, the sensors, which are installed at a density of 15-20 sensors per square mile, need the microphones to be exposed in order to collect the ambient noise.

"It would be fairly straight forward to put [the sensor] in a box and seal it and put it outside," McFarlin said. "But then my microphones wouldn't be able to hear anything."

And so, McFarlin and his team designed the sensor such that the microphone is at the end of the board and extends into an area protected against moisture ingress, but otherwise relatively transparent to acoustic waves. The solution is in some ways similar to how outdoor microphones and speakers are designed, save for ShotSpotter's desire to build longer-lasting products. McFarlin says once a sensor is installed, they don't want to revisit it for eight to 10 years. For that reason, ShotSpotter decided to use industrial-grade components to survive everything from the heat of Arizona and the cold of New York.

"The challenge was designing electronics that could survive outdoors and do it in a way that was low cost to manufacture," McFarlin said.

Many cities have already taken advantage of the technology. The company, which began its tracking system in Brooklyn, has now expanded to 80 cities. While most implement ShotSpotter in urban areas with high levels of crime, Prescott said the company has seen cities expand their coverage over time. ShotSpotter has an active presence in the east part of Chicago, a city that has been decimated by gun violence.

The data suggests ShotSpotter is providing its desired effect. Prescott said customers have reported a 34 percent reduction in gunfire over an 18-month period.

Moving forward, ShotSpotter hopes to expand to more cities, release an enhanced mobile platform and provide private security at colleges like UC San Diego and the Savannah College of Art and Design. ShotSpotter is also working with companies to help provide relief in the case of an active shooter situation.

For now, however, their most successful product remains the gunshot analysis and locater services.

"This doesn't solve [the issue], but it's a piece of the puzzle that law enforcement agencies need." Prescott said. "So we look at ourselves as a partner."
Aric DiLalla is a Northwestern University graduate and freelance writer living in Denver, Colorado. When he's not writing about electronics, he's probably watching football.