ShotSpotter, a Santa Clara, Calif. startup that can help police and other security forces detect and triangulate gunshots in real-time, has raised $7.5 million for sales, marketing, and international expansion, in its fourth funding to date.

The technology ShotSpotter has developed is conceptually simple: A network of audio sensors attached to phone lines or wireless communication devices is spread across an area, generally within a city. When shots are fired, the sensors pick up the sound, identify it as gunfire (as distinct from backfires, fireworks and other loud noises), triangulates on the source, and automatically notify nearby police.

The specifics are a bit more technical, involving patented algorithms and specialized hardware. Several competitors, like the Secures system from Planning Systems, operate in the same space, but ShotSpotter says its technology requires less sensors and is more accurate.

New investor the Westly Group led the round, and was joined by Norwest Venture Partners, Broidy Capital Management, Labrador Ventures, Levensohn Venture Partners, and Claremont Creek Ventures. ShotSpotter’s previous three rounds, including its previous round of $12 million (click for more coverage), bring its total financing to date to $29.6 million. The company says this funding will be its last.