Amid Debates Over Surveillance Tech, City Advocates Have Privacy Concerns About ShotSpotter

Cambridge police rushed to respond to reports of gunfire and fleeing suspects near Kendall Square on Monday afternoon, the latest development in Cambridge’s ongoing battle against gun violence.

As city officials explore new technologies to secure the city, the Cambridge Police Department continues to rely on ShotSpotter — a widely-used but controversial gunshot detection system. While Cambridge has employed the technology since 2014, advocates worry that the technology impedes on residents’ privacy.

ShotSpotter, which is owned by private company SoundThinking, uses audio sensors placed on the tops of buildings or street lights to listen for sounds that may be gunfire.

“The array of sensors that we put in any ShotSpotter coverage area basically capture data on any loud, impulsive sounds that they hear,” SoundThinking spokesperson Jerome Filip said.

While the sensors’ microphones are always listening for potential gunshots, they are not constantly recording. When sensors detect a loud noise, SoundThinking’s algorithm determines whether the sound is a gunshot and then alerts local police departments. Police are provided with a seconds-long audio recording of the sound after it has been flagged by the system.

“That audio snippet helps them understand the number of rounds fired, the sequence, the timing,” Filip said.

The sensors often mistakenly pick up noises like cars backfiring or fireworks. Almost two-thirds of the ShotSpotter alerts in 2025 have been confirmed to be gunshot-related, according to Robert P. Reardon Jr., a CPD spokesperson.

“It is worth noting that the absence of a victim or ballistic evidence does not automatically rule out that a gunshot had caused the alert,” Reardon wrote. “All that this means is that the alert could not be definitively tied to a known shooting incident by the officers who responded to investigate due to a lack of available evidence.”

Still, activists have expressed concern with ShotSpotter’s perceived ability to record conversations.

“It’s this unaccountable private company that is collecting all of our conversations. So I think it's obviously a stupid dystopian idea,” said Mila Halgren, a member of the MIT Coalition for Palestine.

Filip stressed that it is “extremely improbable” that the sensors would pick up conversations from street level.

“For you to have an alert and to begin with, you need to have at least three or more sensors actually capturing that sound because that's essentially how we triangulate and determine the location,” Filip said.

Filip cited a New York University Policing Project audit which concluded that the “risk of voice surveillance is extremely limited.”

ShotSpotter sensors cover roughly a one mile area in the city, though neither CPD nor SoundThinking disclose their locations or how many sensors are in the city.

A document detailing purported locations of ShotSpotter sensors across the nation was leaked to WIRED in February 2024, including at least 25 sensors in Cambridge. The document shows that Cambridge’s ShotSpotter system encompasses land stretching from Harvard to MIT’s campus, but does not operate on either of the two campuses.

When ShotSpotter came to Cambridge ten years ago, it was implemented in areas with historically high rates of gun-related violent crime. The sensor locations have not changed since it was implemented, according to a 2024 CPD report sent to the City Council.

“While we have not put forth any formal proposal to expand the system, it is something we’d be open to exploring with the community,” Reardon said.

While CPD could request to move the sensors, the report highlighted that the high concentration of gun-related violence has persisted in the same area.

Still, advocates stress that ShotSpotter’s implementation in the city is racially biased — claiming that the technology disproportionately covers predominantly Black neighborhoods. Cambridge-based civil liberties group Digital Fourth wrote a 35-page objection to multiple surveillance technologies, including ShotSpotter, to Cambridge’s Public Safety Committee last March.

“In short, if you’re living in a poorer and more diverse area, you’re subjected to ongoing audio recording. If you’re a student living on the Harvard or MIT campus, or a resident of West Cambridge, you aren’t, irrespective of the level of violent crime near you,” Digital Fourth Chair Alex Marthews wrote in the March statement.

City spokesperson Jeremy H. Warnick wrote in an email that ShotSpotter technology is employed across the city, according to an annual report from CPD.

Filip, the SoundThinking spokesperson, defended the ShotSpotter technology in a written statement.

“Critics have raised concerns that ShotSpotter contributes to over-policing, particularly in black and brown neighborhoods,” he wrote. “But customers themselves – not SoundThinking – decide where to deploy ShotSpotter based on objective historical data.”

The exact number of sensors in Cambridge is still unknown, but Sound Thinking generally sells between 15 to 25 sensors per square mile, according to Filip.

Cambridge spends roughly $50,000 on ShotSpotter annually, funded through the federal Department of Homeland Security’s Urban Areas Security Initiative, which designates federal funds to counterterrorism efforts.

Marthews said in an interview that the federal funding “puts a thumb on the scale for the adoption and rollout of new surveillance technologies,” leaving residents and City Councilors in a “really difficult position.”

“They have to come in right at the end of a funding process, at a time when the City Council is being offered free money by the Feds that includes funding for really important things and that they have to accept on an all or nothing basis,” he added.

—Staff writer Matan H. Josephy can be reached matan.josephy@thecrimson.com. Follow him on X @matanjosephy.

—Staff writer Laurel M. Shugart can be reached at laurel.shugart@thecrimson.com. Follow them on X @laurelmshugart or on Threads @laurel.shugart.