POP Center Responses Gunshot Detection Page 2 

Efficacy and Police Uses of Acoustic Gunshot Detection Systems

AGDS has primarily been used in the United States, although some systems have been deployed in South Africa and Latin America.7 The U.S. has high uptake largely because many U.S. cities have concentrated gun-violence problems that exceed levels seen in Europe and Asia. Also, the current high cost of the systems discourages their use in less wealthy nations.

Most U.S. agencies deploy fixed AGDS that either are integrated into computer-aided dispatch (CAD) systems or interface with mobile data terminals (MDTs) or smartphones, facilitating rapid deployment of officers to locations where gunfire has occurred with AGDS is unclear; however, the current market leader of this technology reports active contracts with over 120 U.S. law enforcement agencies, meaning that many of the larger U.S. agencies likely have a system in place.

Police expect to achieve benefits with the deployment of acoustic systems, but these potential benefits are not consistently documented in the peer-reviewed literature. Nonetheless, the advantages of deploying AGDS can include the following:

Faster response time. AGDS significantly reduce dispatch times. Some systems can bypass 911 systems and provide the gunfire alerts directly to officers. Police travel times to crime scenes might be reduced as well if AGDS alerts are assigned as the highest priority calls, since gunfire reported by residents is often assigned a lower priority.8

Victim assistance. A secondary benefit of reduced response times is that shooting victims may receive emergency medical treatment more quickly, either by being treated by police officers at the scene or arriving at a hospital more quickly.9

Accurate locations. AGDS tend to provide accurate locational data,10, † meaning that officers arrive at more precise locations. AGDS typically can provide both a location on a map (through an MDT or smartphone) and a physical address (if dispatch occurs through a 911 center). Vendors encourage users to rely on maps because those locations are slightly more accurate.

† Not all AGDS products have been studied, and the results may vary across platforms.

Enhanced evidence recovery. A secondary benefit of having a more accurate location of gunfire is that police are likely to collect more evidence (e.g., shell casings). How well AGDS enhances evidence recovery is not yet fully understood, although this metric may be overlooked in  assessments.11 AGDS use in Phoenix, though, has led to investigative benefits.12 The police having a more accurate location appears to be connected to increased retrieval of shell casings and an improved ability to perform link analysis of firearms.

Enhanced reporting of gunfire. AGDS data analysis shows that citizens substantially underreport gunfire. Comparing citizeninitiated calls for service for gunfire against AGDS alerts reveals that about 80 percent of gunfire goes unreported by community members. Use of AGDS should therefore lead to more police responses to gunfire.13 However, AGDS alerts involve a smaller percentage of assaults and homicides than do calls for service from residents, suggesting that acoustic alerts may lead to greater reporting of gunfire but not necessarily of gun violence (assaults and homicides).14 Implementing AGDS can reduce community members’ calls for gunfire. It is not clear whether that reduction represents an actual decline or merely a shift in reporting, with community members counting on the AGDS to report the gunfire they hear.15 Faster police response times might result in police arriving before witnesses can place a call to police.16

Reductions in gun violence. For AGDS to reduce gun violence, it must deter people from shooting their guns when they otherwise would. It could do this by leading to the incarceration of frequent shooters or leading to the confiscation of their guns (assuming they could not easily replace them). Awareness of AGDS could also discourage people from using guns if they believe the system will increase their chances of being arrested. Evidence that AGDS deters gun violence is scant thus far. One study of an AGDS linked to pan-tilt-zoom cameras found that it had no effect on crime levels. 17 Two studies of AGDS in St. Louis found that it had no significant effect on crime levels.18 In Cincinnati, however, AGDS did lead to a significant reduction in assaults with firearms.19 Although gun-violence reductions in Denver were attributed to AGDS deployment, comparable communities that did not have AGDS displayed similar declines.20 Some beneficial effects of AGDS on crime were also found in Milwaukee, Wisconsin, and Richmond, California, 21 but not in Wilmington, Delaware.22 In short, the evidence for AGDS leading to crime reduction is mixed. Most studies focus on the response to gunfire alerts to understand the effect of AGDS on gun violence. A data-driven, problem-oriented approach has thus far been adopted in only East Palo Alto, California.23 Results of this experiment found that preventative patrols in gunfire hotspots had no effect on crime levels. However, the study indicated that AGDS was not fully implemented because of shifts in organizational priorities. Implementation of practices and policies varies widely across agencies, though, and better implementation might lead to better results.

Types of Acoustic Gunshot Detection Systems and Their Applications

Multiple vendors supply AGDS, but they all operate with sensors that are designed to detect and report gunfire sounds. Whereas mobile devices and indoor systems are typically used for military and civilian applications, police rely primarily on fixed outdoor systems in which elevated sensors triangulate the location of gunfire.

Fixed systems can be stand-alone or integrate with closed-circuit television (CCTV),‡ ALPR, or even street lighting.§, 24 AGDS typically feed activations directly to dispatchers and can alert police officers’ mobile phones through vendor-supplied applications. Fully mobile acoustic systems also exist,  offering greater flexibility in deployment.25 Mobile surveillance units, for instance, can often be outfitted with a gunshot detection system. However, gunshot detection boards in these units typically need recalibration after each move. In addition, feeding data from mobile systems directly to dispatch or patrol officers may not always be possible.

‡ See Response Guide No. 4, Video Surveillance of Public Places, 2nd Edition, for additional information.

§ See Response Guide No. 8, Improving Street Lighting to Reduce Crime in Residential Areas, for additional information.

Synchronizing AGDS with camera networks may be helpful in some situations, but whether doing so reduces crime is still disputed.26 In addition, limited camera coverage or other technical limitations can reduce the evidence-gathering potential of AGDS. Pan-tilt-zoom cameras, for instance, can be activated to focus on the source of gunfire, but in narrow city streets, sightlines are often blocked, depending on the height and angle of the camera. 27

Numerous AGDS configuration options have police applications. Full-service systems typically require a minimum coverage of several square miles and thus make economic sense for larger agencies with long-standing gun-violence problems. Smaller, portable systems are more suitable for agencies experiencing occasional gunfire hotspots or rapidly shifting gunfire locations. 

Experimental  Applications of Acoustic Gunshot Detection Technology

Although still largely experimental and not systematically implemented by police, acoustic surveillance can detect far more sounds than just gunfire. Sensors exist that monitor nightlife for sounds that indicate a fight may be developing,28 detect screams, or even monitor illegal logging operations.29 Additionally, sound-detection technology is available to detect breaking glass and car accidents.30 Acoustic cameras provide an additional way to monitor sound. These listening devices are overlaid with video feeds and translate sound into visual heat maps. This technology can detect such things as lights and fans in illegal marijuana growing enterprises as well as aggressive voices on streets. Acoustic cameras can be integrated with other surveillance systems and can even adjust street lighting if fights appear to break out.31 Commercial and police adoption of such systems remain limited at this time.

SMART Policing Initiative (SPI) in Wilmington, Delaware: Targeting Violent Crime. 32

The National Police Foundation (now known as the National Policing Institute) evaluated the AGDS expansion in the Wilmington, Delaware, Police Department. In addition to expanding the coverage area of the AGDS, the police also integrated its acoustic system with CCTV, meaning that gunfire detection prompted nearby cameras to pan and zoom to that location.

Wilmington initially began using AGDS in 2013 and received a Smart Policing Initiative (formerly known as Strategies for Policing Innovation) grant in 2018 to increase the efficacy of AGDS in multiple areas. The objectives of the grant were as follows:

(a) Linking CCTV with AGDS to promote a faster response

(b) Providing better evidence recovery (e.g., casings) from offenders

(c) Increasing the clearance rate for firearm-related crimes, deterring gun violence, and improving public perceptions of police

Project planning began in 2019, with much of the technological upgrades being completed in 2020. Police survey responses about the technology were positive, reporting that it enabled quicker  identification of a crime scene, enhanced evidence collection, facilitated prosecution, and provided faster aid to victims. Though the integration of cameras was viewed positively, police indicated that the zoom function sometimes hindered more than it helped. In addition, data analysis revealed that evidence recovery increased, but case resolution did not, and gun-related crimes increased post-implementation (the authors indicate that the latter findings could have been affected by the COVID-19 pandemic). 

Accuracy of Acoustic Gunshot Detection Systems

Accuracy can refer to whether the system “hears” gunshots and accurately distinguishes gunfire from other loud noises, such as fireworks, vehicle backfires, and construction noises (see Table 1). A gunshot detection system functions accurately when an identified gunfire alert is truly gunfire (true positive). At times, a system may misidentify other sounds for gunfire (i.e., a false positive). On other occasions, a system may miss or misclassify true gunfire (i.e., a false negative). False positives are a nuisance because they prompt a police response to a non-gunfire sound, but false negatives are more damaging because they fail to prompt a police response to actual gunfire. The rates of false positives and false negatives are unknown but likely depend on a particular system’s configuration. Some systems retain data for alerts that are not deemed gunfire, meaning that investigators can still derive information from the system even though there is no immediate response, which might assist with evidence retrieval and crime-scene identification.

Accuracy can also refer to the precision with which the system can determine the location from which the gunshot was fired. Although hard standards for geographical accuracy do not exist, it is reasonable to expect a system to locate most gunfire incidents within an average spatial error margin of about 41 feet (12 meters). 33

Because of the variety in AGDS, estimating their average accuracy is difficult. Their detection accuracy can also be affected by environmental conditions (e.g., snow, wind, thunder), topography, foliage, and the built environment. Accuracy is also somewhat dependent on the amount of gunfire in an incident, since fewer shots are more likely to go undetected than many shots fired in rapid sequence. 34 The caliber of the firearm also appears relevant, with lower calibers less frequently identified. 35

AGDS pick up most gunfire with high accuracy. Both live-fire tests and agency findings demonstrate a detection accuracy of around 80 percent, ranging from 70 percent in St. Louis to 90 percent in Las Vegas.36 As a result, roughly 20 percent of cases of true gunfire produce a false negative.

The proportion of false positives AGDS produce is unclear. Some have argued that the majority of AGDS calls for service are false positives,37 which is almost certainly untrue. Some gunshot detection systems will identify sounds that resemble gunfire, and such systems may have difficulty filtering false positives, but the systems currently in use by the majority of large police agencies include incident reviews, which reduce false positives. 38 Certainly, some false positives are to be expected, especially during fireworks holidays, but no good method for assessing the relative frequency of false positives exists. Responding officers might be unable to determine what caused the sound detected by the AGDS. If the noise at the source location has ceased or if other evidence of gunfire, such as shell casings, has been removed prior to the arrival of officers, the true source of the noise might never be known. The same challenges exist for gunfire calls for service from the public. When police do suspect recurrent false positives, they should discuss them with their system vendor to help them improve the system’s accuracy. A high false positive rate may explain why some early adopters of the technology have abandoned the technology, since it can discourage officers from responding promptly or investigating thoroughly.39 At present, improved algorithms and human review of incidents appear to have reduced this issue.40

The geographical accuracy of AGDS may also vary by system, but most incidents are accurately pinpointed.41 One system, for example, uses an 82-foot (25-meter) cone of uncertainty in locating gunfire incidents, indicating great confidence within this range.42 The distance from the nearest sensors matters in the geographical accuracy of the alerts; gunfire within 500 feet of sensors will likely be more accurately pinpointed than gunfire that occurs 2,000 feet from sensors. As a result, the accuracy of a system is partly determined by the spacing and sensitivity of the sensors. However, the difference between geospatial coordinates and address-based locations is sometimes misunderstood. 43 Most acoustic systems will forward two location descriptors: latitude and longitude and a physical address. The physical address is typically based on parcel data, which can create unavoidable challenges when relaying locations to officers. Large parcels, such as parks or housing complexes, often are represented in address databases with a single address, which means the  actual location of the gunfire may be some distance from the reported address. For this reason, police should respond to the geospatial coordinates on a map when possible. The implications of this issue for police response are described later in the section Responding to Gunshots.

Table 1. Acoustic Gunshot Detection System Error Types