How it works

QUINCY — ShotSpotter Flex measures tiny differences in the time it takes for the sound of a shot to reach a series of sensors. Software is then used to triangulate the point of origin.

“When a sound goes off, it ripples out like when you throw a stone into a pond and the waves ripple out in a circle,” said Lydia Barrett, vice president of marketing and communications for SST, the Newark, Calif.-based company that holds Quincy’s contract for ShotSpotter Flex.

Sensors are placed in high locations, usually rooftops, Barrett said. Between 15 and 18 sensors are placed inside a square mile. Usually, sensors are set up in areas known for high crime rates. It would be unusual for sensors to be set up throughout an entire city, she said.

Barrett and Quincy Police Chief Richard Ackerman declined to say where in Quincy the sensors are located, saying they did not want the sensors to become targets of vandals. They said they also did not want criminals knowing what areas of town are covered.

Within the first few seconds after a shot is fired, information from the sensors is relayed to gunfire and acoustic experts at the company, Barrett said. Within 20 to 40 seconds, they analyze the data for type of weapon and for possible direction of travel, then relay that information to computers in Quincy patrol cars.

Information comes in to officers with an address, latitude and longitude readings, and a description of the gunfire from the company experts, Barrett said.

— Dee Riggs, World staff