For the past eight years or so, the ShotSpotter gunshot-location technology has helped police departments improve response times for gunshot incidents outdoors. Residents often mistake a gunshot for other loud noises—such a truck backfiring—and don’t bother calling 911, which greatly hinders response and the ability to track down the perpetrators.

However, the ShotSpotter system is able to discern whether the noise is an actual gunshot; when it is, the system automatically notifies the closest PSAP and provides the location information, which is then transmitted to patrol officers. The system is quite accurate regarding location information, because it triangulates data gathered from 15-20 sensors installed per square mile and uses GPS to synchronize the sensors, according to Gregg Rowland, senior vice president of sales and marketing for Newark, Calif.-based SST, which developed Shotspotter.

“When something goes boom or bang, the boom or bang arrives at our sensors at a different time,” he said. “We put 15-20 sensors per square mile on top of buildings and light poles and wherever, and we use time of arrival to determine the actual location of the gunfire by triangulating on the sound.

“GPS has the most accurate clock possible, and one of our core patents is using the GPS clock to measure the time of sound arrival.”

With the increase of active shooter incidents over the last few years—at schools and universities, shopping malls and other facilities where large amounts of people can be found—the company’s customers have been clamoring for an indoor version of the ShotSpotter platform. Last November, SST introduced its SiteSecure solution that provides the same sort of gunshot-detection capability indoors and integrates with the outdoor platform to create a “protective bubble” around the facility, according to a company press release.

SST encountered several challenges in bringing its outdoor technology indoors, including the fact that gunfire sounds very different indoors than it does outdoors, according to Rowland.

“We have the largest database in the world of gunshot sounds outdoors, but we really needed to characterize and tune our software to recognize what it sounds like to shoot a gun inside,” Rowland said. “Our biggest ‘Aha’ moment was when we discovered that these buildings are designed to dampen sound—they don’t want screaming kids in one classroom to disrupt what’s going on in the classroom across the hall.

“So, you can close a classroom door and shoot an automatic weapon in the hallway, and people aren’t going to hear it, and if they do, they won’t recognize what it is,” he continued. “It sounds like someone is slapping their hand on the wall.”

SST addressed this by staging test events at several schools—when students weren’t present—during which they fired different firearms and calibers to accurately capture the sounds.

“We used all sorts of scenarios–in classrooms, hallways and gyms, doors open and doors closed, you name it,” Rowland said.

Another challenge encountered by SST is that some states prohibit open microphones from being used indoors for privacy reasons. To overcome this, SST designed its indoor sensors to “feel” a gunshot when it occurs.

“There is nothing louder in a room than somebody shooting a weapon,” Rowland said. “So, we use sound-pressure levels. The sensors feel that pressure, and there’s nothing in the building that can create that same pressure, other than a gun. So we can determine location by identifying what sensor or sensors felt that sound pressure.”

Perhaps the biggest hurdle that needed to be overcome was cost—the indoor solution needs a lot more sensors, and schools typically aren’t flush with cash these days—a problem that is compounded at colleges and universities, which typically have many buildings on their campuses. SST needed to find a way to reduce the cost of the system, particularly the sensors. Greatly improved economies of scale helped in this regard, according to Rowland.

“When we first started, our sensors cost a lot of money to build, and we produced them in very small runs,” he said. “A customer might build out over two, three or four square miles, so he’d buy 50 of them. Now we’re building hundreds and thousands of them.

“Also, the technology inside the sensor that used to cost a lot of money now is much lower, because many of the components are the same as what you’d find in a cell phone or other high-commodity things. Our sensors literally are little computers now.”

With cost containment in mind, SST also switched to 4G commercial technology—from private radio and unlicensed point-to-multipoint technology—to transmit the data generated from its outdoor sensors to 911 centers. The indoor system uses a modified Zigbee wireless mesh network to communicate to the outdoor sensors, which then backhauls the data to the 911 center.

“From a cost/reliability standpoint, we have several thousand sensors out there right now that are on 4G, and it’s actually been more reliable than a private network,” Rowland said. “Long-term, our clients have trees that grow up, and that causes foliage-related interference problems.  Also, wind hits the antennas and messes up the array.

“We don’t experience any of that stuff with 4G … which also costs about one-tenth what it would cost to build a private or unlicensed network.”

According to Rowland, costs for the indoor system range from about $5,000 to $100,000 for a large college. He said that SiteSecure is less expensive than a fire-alarm system and hopes that it becomes just as ubiquitous.

“Over the last 20 years, no student has been killed in a school by a fire, but over the same time period more than 200 students have been killed by firearms in schools,” he said.