A low-cost clone might not be feasible due to patents: http://www.shotspotter.com/patents
A low-cost clone might not be feasible due to patents: http://www.shotspotter.com/patents
Everytime it happens the first thing the neighbors do is hop on the local listserv and literally triangulate the source of the shooting based on where they heard the sound coming from, with respect to each other.
A publicly accessible / open source / subsidized / non-locked down / crowd sourced version of ShotSpotter would make a lot of sense. I would hope for something more along the lines of a Safecast[1] for gunshots. If one can cross-correlate with traffic footage it should have a chilling effect. That the ShotSpotter system is locked down and costs ~$4,166.66 per sensor is madness [2].
[2] http://www.wired.com/wired/archive/15.04/shotspotter.html
I'd worry about the liability from false positives with a active response system. Even something as limited as locking the doors to a room to contain the shooter could lead to liability from someone being trapped with the shooter.
If you assume a large indoor setting, something like a mall or school, maybe combine the detection network with a evacuation guidance system that leads people away from the incident. Mass shootings are pretty rare so maybe you could use it as a general "smart evacuation" system. OTOH there's probably a crowd psychology problem in there that makes that unfeasible.
If the device has image recognition to detect firearms being brandished, the operator could trigger the defenses before the first shot is fired perhaps.