Acoustic Gunshot Detection 

Acoustic gunshot detection is a system designed to detect, record, and locate the sound of gun fire and alert law enforcement. The equipment usually takes the form of sensitive microphones and sensors, some of which must always be listening for the sound of gunshots. They are often accompanied by cameras. They are usually mounted on street lights, or other elevated structures, though some mobile and fixed systems operate in both indoor and outdoor settings.

Police and companies that manufacture and sell acoustic gunshot detection systems have claimed that the point of this technology is to inform police of the location of shots fired, more quickly and accurately than relying on witnesses who overheard gun fire and who may call the police. However, we now know that gunshot detection systems can also hear and record human voices¾and police have used these recordings as evidence in court. As is so often the case with police surveillance technologies, a device initially deployed for a less intrusive purpose (here,  to report the sound and location of gunshots to police) is now being used for a far more intrusive purposes (to spy on people having conversations within the vicinity of their sensitive microphones).

How Acoustic Gunshot Detection Works

ShotSpotter minneapolis

Minneapolis Shot Spotter Activation Call Locations in 2011. Source: minneapolismn.gov

Acoustic gunshot detection relies on a series of sensors, often placed on lamp posts or buildings. If a gunshot is fired, the sensors detect the specific acoustic signature of a gunshot and send the exact time and location to the police. Location is determined by measuring the amount of time it takes for the sound to reach sensors in different locations.

According to ShotSpotter, the largest vendor of acoustic gunshot technology, this information is then verified by acoustic experts to confirm the sound is gunfire, and not a car backfire, fire crackers, or other sounds that could be mistaken for gun shots. The sensors themselves can only determine whether there is a loud noise that somewhat resembles a gunshot. It’s still up to people, sitting and listening on headphones, to say whether or not shots were fired.

With microphones running and recording during the moments that it captures any sounds resembling gunshots, acoustic gunshot detection also records other sounds, including human voices, that occur within the vicinity of its microphones during a suspected shooting incident.

Who Sells Acoustic Gunshot Detection

Spotshotter SEC Filling

Mapping a potential gunshot on ShotSpotter's system. Source ShotSpotter SEC filing.

ShotSpotter is by far the largest vendor of acoustic gunshot detection systems. Currently, 100 cities are using ShotSpotter in the U.S., South Africa, and the Bahamas. The company earned $34.75 million in revenue in 2018, and in March 2019, ShotSpotter announced it would be offering publicly traded stock.

Raytheon produces the Boomerang system, which detects small arms fire. It can be mounted on a vehicle and has been used in U.S. warzones like Afghanistan. Other startups, such as Aegis, rely more heavily on artificial intelligence, and visual gun recognition, in recognizing potential shooting situations.

Threats Posed by Acoustic Gunshot Detection

Even if aimed at gunshots, this technology also captures human voices, at least some of the time. Yet people in public places ¾for example, having a quiet conversation on a deserted street¾are often entitled to a reasonable expectation of privacy, without overhead microphones unexpectedly recording their conversation. Federal and state eavesdropping statutes (sometimes called wiretapping or interception laws) typically prohibit the recording of private conversations absent consent from at least one person in that conversation.

In at least two criminal trials, the California case People v. Johnson and Commonwealth v. Denison, in Massachusetts, prosecutors sought to introduce as evidence audio of voices recorded on Acoustic Gunshot Detection system. In Johnson, the court allowed this. In Denison, the court did not, ruling that a recording of “oral communication” is prohibited “interception” under the Massachusetts Wiretap Act.

Moreover, acoustic gunshot detection systems are often placed in what police consider to be high-crime areas. As with many police surveillance systems, this can result in excessive scrutiny of the neighborhoods where people of color may live. 

There also are concerns about the accuracy of Shot Spotter. As Dana Delger, an attorney for the Innocence Project told the Democrat & Chronicle in regards to the placement of Acoustic Gunshot Detection in Rochester, “At the end of the day this is a machine that basically can tell you that there was a loud sound and then a human has to tell you whether it was gunfire or not.”

In one 2017 case, a ShotSpotter forensic analyst testified that the sensors incorrectly placed the location of a shooting a block away from where it actually occurred. When asked about the company’s guarantee of accuracy, the analyst said, “Our guarantee was put together by our sales and marketing department, no our engineers.”

EFF’s Work on Acoustic Gunshot Detection

As part of its measures to raise awareness about the invasive uses of surveillance technologies, EFF has long raised concerns about  acoustic gunshot detection systems recording the conversations of unsuspecting people on the street and in other public places. 

For More Information:

 Shotspotter: gunshot detection system raises privacy concerns on campuses- The Guardian.

There’s a Secret Technology in 90 US cities that listens for gunfire 24/7 – Business Insider