Sound Surveillance Locates Shooters

June 2003
By Sharon Berry

Dispersed sensors pinpoint the location of gunshots, provide wireless notification of incident details.

The Washington, D.C., Beltway sniper shootings, military campaigns abroad and steadily increasing terrorist alerts have inspired the creation of a law enforcement tool that promises to improve security in local jurisdictions and on the battlefield.

The On Alert Gunshot Detection System (GDS) can be embedded in high-crime or battle areas and quickly responds when it detects gunfire. When the system recognizes the sound of a gunshot, it immediately relays precise details to authorities, including the type of gun that was used, the number of shots fired and the location from which the shots were fired. Rapid dissemination of this information reduces threat escalation and assists security efforts in regions under surveillance.

GDS combines acoustic sensors, transmit/receive components, computer chips, an integrative motherboard and a power source into a wireless package. The technology is packaged in two models, one for civil authorities and the other for military personnel. Proxity Digital Networks, New Orleans, is developing the system under a licensing agreement with Synchros Technologies Incorporated, Bixby, Oklahoma.

System sensors, known as sleepers, activate only when they perceive the distinct signatures of gunshots. Every weapon has a unique signature just like a fingerprint, says William C. Robinson, chief executive officer of Proxity. The company is recording the sounds from different caliber weapons and is beginning to catalog weapons and their corresponding signatures. The sensors are programmed to distinguish specific sounds. “This type of sensor is what we think of as an enhanced human ear,” Robinson shares. “It gives more details about an event than can someone who hears a noise, calls the police and says, ‘I heard a gunshot.’ The GDS gives authorities enough information to respond in an organized manner.”

At least three sensors are required for the GDS to pinpoint the location of weapons fire. Once a sound is recognized, the system triangulates a shot’s origin to within 3 to 5 feet. Via a connection to the Internet, On Alert accesses the weapons database that resides on a secure server, matches the signal and disseminates the specifications and location to a 911 call center or to a handheld personal digital assistant (PDA). Engineers have built a software program that users can install on most off-the-shelf PDAs. The software converts the information into a real-time map and shows the event location along with other incident details on a grid.

Robinson notes that the system can be programmed on-the-fly to recognize specific sounds. “We can record the sound and, via the Internet, upload the new audio information to the sensor chip,” he explains. “In the instance of the D.C. Beltway snipers, there was a particular type of gun used. If the authorities found a shell casing or other forensic evidence, we could immediately highlight the corresponding weapon and tell the sensor to listen for it.”

The mechanism for transmitting information from the GDS is still being developed. Proxity and Synchros are working with several major telecommunications carriers, satellite providers and wireless integrators to develop their approach.

The civilian model in development is a small, wireless self-contained unit that is powered by lithium batteries and backed up by solar power when the batteries recharge. The sensor prototypes measure 14 inches long, weigh approximately 2 pounds each and are designed for omnidirectional coverage estimated at 400 yards. Proxity is working to reduce the sensors to the size of a pager.

On Alert can be clipped to power lines, hung from street lights or placed in trees. “The system is designed so that it could be dispersed in a matter of hours,” Robinson explains. “And it is up and running as soon as you turn it on. If you have an event such as the Super Bowl or a presidential speech, you want to protect the area rapidly but temporarily. Once the event is over, the system can be taken down easily.”

A military version also is being designed for deployment on the helmets of individual soldiers as they enter the field. This variant would require someone behind the lines to monitor the location of the troops and triangulate hostile fire to help troops locate and take out the source of weapons fire, Robinson says. These systems currently are designed as handheld units that can be clipped to a belt.

The technology is being tested in Tulsa, Oklahoma, with plans to finalize the civilian model this quarter. “We hope to be in live situation testing soon,” he says. “We currently are negotiating to conduct tests with the help of civilian and federal authorities, including the U.S. Department of Justice, U.S. Coast Guard, all of the military services and—on the civilian side—with private security firms and local police departments. We have talked to the military and, because of what is going on overseas, have accelerated the military version,” Robinson reveals. The testing phase will take approximately three to six months.

Among the challenges of developing On Alert is filtering out background noise such as false signatures—fireworks or the backfire of a car—or echoes off buildings, which make it difficult to pinpoint the location of the sound. But Proxity’s engineers believe that carefully placing sensors and monitoring the intensity of a signal according to each sensor could help separate false and true signatures. The team is developing acoustical wave technology that will allow faster installation, a more random layout of sensors in a grid and the use of terrain to help detect events. 

Building a comprehensive weapons database also is a challenge. “We need to collaborate with a law enforcement or military agency that has a large inventory of weapons and sit down and painstakingly record all of the caliber of weapons,” Robinson says. “We’ve been negotiating with a couple of sources. Once that’s in place, we can fine-tune the system and make it much more accurate. As we build our audio inventory, ultimately the GDS will be able to detect the caliber of weapons and possibly even match the caliber to the type of weapon based on the harmonics of the bullet exiting the barrel. Individual weapons have unique signatures because of the length of the barrel, the density of the metal and the caliber of bullet.”

Looking into the future, Robinson shares that modifications to the On Alert GDS may allow for more civilian uses. Any event that has a predictable acoustic, subsonic or ultrasonic signature could be detected and located with the technology. Next-generation versions could capture screams in parking garages or sounds from natural events, including tornadoes and earthquakes.

On Alert also could be integrated with surveillance systems, infrared cameras, satellite imagery or other visual technology that captures events. The system could cue the other integrated systems to review the recordings and save them for further analysis. For example, when a programmed sound signature is recognized, a camera would turn on and record in the direction of the sound, giving authorities more comprehensive information.

“We feel that seeking out ways to combat crimes is a big contribution to the citizen as well as to the military,” Robinson says. “Anything we can provide to protect our military, we feel we’re making a contribution to clean up the world.”


Additional information on the On Alert Gunshot Detection System is available on the World Wide Web at and

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