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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.


The key to time-reversal for an active shooter detection/tracking application is being able to estimate the space-time transfer function (Green’s function) between source-enclosure-receiver.  This approach begins with the acoustic mapping of an indoor muzzle blast. 

This is accomplished with a synthetically generated sound that accurately mimics a discharged firearm.  The synthetic sound is used to calibrate the indoor environment so that deployed sensors can interpret and locate the origin of a gunshot, i.e. the location of a shooter in an event can be identified.   Additionally, with the building or space thoroughly ‘mapped’, a shooter can be “tracked” by subsequent discharges of the firearm within the building.  The technology is relatively low cost and meets the accuracy required for emergency response systems.


The sensor network was developed to be low cost and extremely accurate. These are two major requirements for commercialization success. No other technology has been commercialized that can meet the accuracy while also inexpensive to deploy. This should enable budget-constrained public schools, universities, commercial and government building managements to acquire this technology to improve safety in their indoor facilities.

LLNL’s technology has numerous advantages:

  1. Accurate locator
  2. Tracking ability
  3. Low cost
  4. Accurate sound discrimination
Potential Applications

LLNL’s novel approach can accurately locate and track active shooters in indoor spaces and has been designed to be installed in many types of buildings (e.g. public school, universities, government, commercial/retail, transportation centers like airports). 

It can provide first responders with near real time information of the moment a shot was discharged, its location within a building and when the shooter has moved and discharged a gun in a new location.

Development Status

Current stage of technology development:  TRL 8

US Patent No. 11,495,243 Localization based on time-reversed event sounds published 11/08/2022 (

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