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While SoundThinking says its alerts are reviewed by its Incident Review Center before being sent to the police, in Pasadena, officers who investigated ShotSpotter alerts reported that the suspected gunfire was sometimes something else entirely: a car backfiring, construction noise, or fireworks, Knock LA reported.

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The document provides a detailed snapshot of the company’s operations at the specific point in time that the file was created. We do not know if the sensors described in the document are still operational or not.

The gunshot-detection system ShotSpotter has for years drawn criticism from activists and academics who believe the company behind the system, SoundThinking, places its microphone sensors primarily in low-income communities of color. Now, a WIRED analysis of data leaked from the company reveals the secret locations of ShotSpotter sensors around the globe and the US communities most directly impacted by the surveillance.

Finding shell casings can be extremely difficult. A Los Angeles Police Department officer not authorized to speak to the media tells WIRED they’ve spent “hours” searching for bullet casings. Just because officers don’t find evidence of gunfire, they say, doesn’t mean it didn’t happen.

Additional data analysis by Matt Casey, data science content lead at Snorkel AI, a firm that helps companies with AI projects and builds custom AI with its data development platform.

1. Definition of a Convex Mirror: A convex mirror is a type of spherical mirror. It is characterized by its reflecting surface, which is curved outward.

But what happens on the ground often paints a more complicated picture than what Chittum describes. WIRED reviewed body camera footage and police records of a 2022 ShotSpotter arrest in Cincinnati. According to the records, at 8:21 pm on New Year's Eve, police officers were dispatched to an area where two loud sounds were picked up by SoundThinking sensors. When the officers arrived, they quickly detained a tall man in a blue hoodie and black jacket who was standing near the corner where the technology had indicated gunfire.

“I wish we were deployed everywhere,” Chittum says. “If you can only deploy a tool on a limited basis, of course, you deploy it in the place where it could do the greatest good.”

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While Chittum concedes that the company may occasionally miss gunshots, he argues that the presence of thousands of broken sensors doesn’t necessarily make the system unreliable. To ensure total coverage of an area, Chittum says, the company will strategically “oversaturate” an area with sensors. For a sound to be flagged as a gunshot, Chittum says, at least three sensors need to detect it; typically, Chittum says, a gunshot triggers a dozen or more sensors. “If one doesn’t catch it, others will,” he says.

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In an interview, Tom Chittum, senior vice president of forensic services at SoundThinking, tells WIRED that he is “willing to accept that our findings are true” and confirmed that the document is likely authentic. A former associate deputy director of the US Bureau of Alcohol, Tobacco, Firearms and Explosives, Chittum argues that SoundThinking sensors provide police departments with a commonsense resource to help combat gun violence. He claims that SoundThinking places the sensors in areas that are likely disproportionately impacted by gun violence, which helps police quickly get to the scene, especially when no emergency call is made to 911.

Until now, the exact locations of SoundThinking’s sensors have been kept secret from both its police department clients and the public at large. A leaked document, which WIRED obtained from a source under the condition of anonymity, details the alleged precise locations and uptime of 25,580 ShotSpotter microphones. The data exposes for the first time the reach of SoundThinking’s network of surveillance devices and adds new context to an ongoing debate between activists and academics who claim ShotSpotter perpetuates biased policing practices and proponents of the technology.

An analysis of sensor distribution in US cities in the leaked data set found that, in aggregate, nearly 70 percent of people who live in a neighborhood with at least one SoundThinking sensor identified in the ACS data as either Black or Latine. Nearly three-quarters of these neighborhoods are majority nonwhite, and the average household earns a little more than $50,000 a year.

SoundThinking’s secrecy around the location of its sensors has long been a point of contention for activists and researchers. By 2019, at the recommendation of the Policing Project, a public safety nonprofit run out of New York University School of Law, the company adopted a policy that formally states that its clients cannot access the precise locations of their gunshot detection equipment. According to Chittum, the company will even resist subpoenas for information about the location of their sensors in court. The reason for this, according to Chittum, is for the security of the people who cooperate with SoundThinking as well as to protect their sensors from vandalism.

WIRED vetted the locations listed in the document by visiting sensors in two cities: Pasadena, California, and Newburgh, New York. WIRED also used Google Street View to virtually visit a random sample of locations listed in the document and found they matched the placement of equipment that appears to be SoundThinking’s.

2. Curvature: The outward curvature means that the surface of the mirror bulges away from the object being reflected. This is in contrast to concave mirrors, which have a reflecting surface that curves inward.

In January, South Side Weekly reported that SoundThinking sensors in Chicago failed to issue an alert for a 55-round shooting at a gyro shop that wounded two men. According to internal company emails reviewed by the publication, the cause of the missed detections were three out-of-service sensors. According to the file, as many as 357 sensors in the Chicago Metropolitan Area were broken, unreliable, or out of service at the time the file was created—9 percent of the total.

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According to SoundThinking’s Chittum, the idea that police show up to ShotSpotter alerts ready to make arrests is speculation based on a few high-profile incidents. Instead, he argues that ShotSpotter provides law enforcement with accurate data to engage the community safely. “It allows police to knock on a door and tell residents, ‘Hey, we got a report of gunfire, we are just checking to see if everyone is OK. Did you hear anything? Did you see anything? If you do, please call us; we care, and we’ll come.’”

Updated 2/23/2024, 12:15 pm EST to clarify that the list of sensors included in the document WIRED obtained may not include every ShotSpotter microphone.

More than 12 million Americans live in neighborhoods with at least one ShotSpotter sensor, according to a WIRED analysis of the document’s sensor locations. According to the file, which includes the geographic coordinates of each ShotSpotter microphone, sensors can be found in 84 metropolitan areas and 34 states or territories in the United States. Nine cities have more than 500 sensors installed, including Albuquerque, New Mexico; Chicago, Illinois; Washington, DC; San Juan, Puerto Rico; and Las Vegas, Nevada. The document does not indicate whether the list of sensors is comprehensive.

Chris Baumohl, an EPIC Law Fellow and coauthor of the petition to the DOJ, tells WIRED that our findings confirm what the nonprofit wrote in their petition in September: that ShotSpotter surveillance disproportionately occurs in communities of color. He also alleges that the technology primes police to go into minority communities believing that shots are fired, whether accurate or not. The result, Baumohl argues, is that community members are more likely to be picked up on bench warrants, misdemeanors, and for other reasons unrelated to guns.

According to police records, there were nine officers on the scene that night. Body camera footage shows one of the officers rifling through the man’s pockets as others milled around. Some pointed their flashlights at the ground or in the windows of parked cars. Others chatted, speculating about the potential whereabouts of bullet casings.

In marketing material, SoundThinking claims that its gunshot-detection system has a 97 percent aggregate accuracy rate over a two-year period. Experts question those figures. A 2021 study conducted by the MacArthur Justice Center at the Northwestern University School of Law found that over two years, 89 percent of ShotSpotter alerts in the city did not lead to police finding evidence of a gun-related crime on arrival, such as shell casings. Recent investigations by both Knock LA and Houston Chronicle, have similarly raised doubts about the system's effectiveness. Both reports found that gunshot alerts triggered by the software largely resulted in dead ends for police.

6. Visual Representation: To better understand, one can visualize a diagram where the outward-curving surface of the mirror is shown, indicating how light rays behave when they strike the mirror.

Looking at individual jurisdictions, the pattern appeared the same. Winston-Salem, North Carolina, for example, is approximately 43 percent white, but only 13 percent of the residents who live in areas that have at least one ShotSpotter device installed identify as white, according to ACS estimates. In Fort Myers, Florida, roughly 19 percent of the population is Black; in block groups with at least one sensor, however, approximately 41 percent of the population is Black.

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Police never found a bullet casing, gun, or bullet hole. They arrested the man anyway. After running his name through their on-car computer, they discovered he had warrants out for his arrest. He had failed to appear in court for traffic violations.

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4. Spherical Part: A convex mirror is essentially a segment of a sphere. The part of the sphere that forms the mirror is the outer surface, which is the reflecting surface.

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Ultimately, Chittum argues, ShotSpotter is simply a tool. When used correctly it can help police-community relations. “It’s up to the police to decide how they use it,” he says.

What is the difference between the virtual images produced by (i) plane mirror (ii) concave mirror and (iii) convex mirror ?

Derive mirror equation for a convex mirror. Usingit, show that a convex mirror always produces a virtual image, independent of the location of object.

According to Chittum, SoundThinking’s sensor deployment is not informed by race. The company asks police departments who purchase its system for data about gun violence, which Chittum characterizes as objective, to recommend a general plan for sensor placement. Once a plan is agreed upon by the department, the company will seek permission from private property owners, utility companies, and businesses to install sensors on their premises. While Chittum says that most property owners agree to do so without needing incentivization, the company will occasionally offer owners gift cards to secure access to their private property. According to SoundThinking, police are never told the precise location of the sensors that the company ultimately installs.

According to the document, SoundThinking equipment has been installed at more than a thousand elementary and high schools; they are perched atop dozens of billboards, scores of hospitals, and within more than a hundred public housing complexes. They can be found on significant US government buildings, including the headquarters of the Federal Bureau of Investigation, the Department of Justice, and the US Court of Appeals in Washington, DC.

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Using population estimates from the most recent five-year American Community Survey (ACS), WIRED collected demographic information from every Census block group—clusters of blocks that generally have a population of between 600 to 3,000 people—with at least one sensor.

3. Diverging Mirror: Convex mirrors are also referred to as diverging mirrors. This is because they cause parallel rays of light that strike the mirror to diverge (spread out) after reflection.

An object is placed at a distance 48 cm from a convex mirror. A plane mirror is placed in between at a distance 16 cm from convex mirror. It is found that virtual images formed by both the mirrors coincide. What is focal length of convex mirror?

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According to the file, 2,680 of SoundThinking sensors—one in 10—were categorized as either broken, unreliable, or out of service at the time the file was created, allegedly late last year.

In September last year, a leading US-based civil liberties nonprofit, the Electronic Privacy Information Center (EPIC), wrote a letter to the US Department of Justice, asking the agency to investigate whether cities using ShotSpotter are violating the Civil Rights Act. Citing news reports and academic studies, EPIC alleged that Sound Thinking’s sensors are directing law enforcement to over-police areas where the fewest number of white residents live.

In February, a leaked internal report from the State's Attorney’s Office in Illinois’ Cook County, where Chicago is located, found that nearly a third of arrests stemming from a ShotSpotter alert had nothing to do with a gun, Baumohl points out. On February 13, Chicago mayor Brandon Johnson, a vocal critic of ShotSpotter, said the city won't renew its contract with SoundThinking.

5. Cartesian Coordinate System: In the Cartesian coordinate system, the reflecting surface of a convex mirror is considered positive on one side and negative on the other. This helps in understanding the properties of the mirror in terms of optics.