Determining the exact location of an indoor shooter(s) has been a technical challenge for companies that develop sensors intended for law enforcement and national security applications.  And with the increased number of high-profile mass shootings, whether in schools, military bases, or shopping malls, there is an urgent need for more accurate indoor shooter detection systems that are also low-cost since many institutions that would benefit are budget-constrained.

False alarms could cause an emergency evacuation and armed response.  Not only is there a risk for accidental shooting by law enforcement to a nonevent, having multiple false positives would undermine overall confidence in the system (e.g. building occupants become complacent to an alarm event).  Another major challenge is indoor places are comprised of enclosed spaces, which inherently have obstructions.  These impediments (e.g. walls, ceilings, intersecting hallways, furniture) cause any propagated sound or signals to bounce and bend, spreading then in a multitude of directions while also distorting both their shape and amplitudes, thereby making localization extremely difficult. 

Presently, there are few successful technologies available in the market that could detect and locate the shooter(s) indoors in a real-time manner, which could enable first responders to respond quickly and mitigate further violence. However, LLNL researchers have developed a novel approach to accurately retrace acoustic sounds even in cluttered indoor environments using time-reversal focusing.  Time-reversal is a simple concept that many of us have observed when viewing a movie of a building demolition; running the movie in-reverse allows us to visually reconstruct the building.  Similarly, time-reversal can be applied to “unravel” propagating acoustic (source) signals due to inherent obstructions in an enclosed environment.