Such a system is already in place in various cities, as outlined last May by The New York Times, and it records a whole lot more than gunshots. A sergeant for the Richmond Police Department told the Times he could hear, “doors slamming, birds chirping, cars on the highway, horns honking."

These systems can also record conversations, which raises questions about the limits of police surveillance. Indeed, one murder case in New Bedford, Mass. is expected to hinge on a recorded argument, according to the Times.

The main supplier of the current system is ShotSpotter, which lists Lockheed Martin and the Ferguson Group as two of its three Strategic Partners.