Gunshot recognition system aims to counter crime

CHICAGO — Police installed video surveillance cameras around town and saw Chicago's murder rate fall to its lowest level in four decades. Now they hope to further cut crime by not only watching but listening.

The city is employing new technology that recognizes the sound of a gunshot within a two-block radius, pinpoints the source, turns a surveillance camera toward the shooter and places a 911 call.

"Instead of just having eyes, you have the advantage of both eyes and ears," said Bryan Baker, chief executive of Safety Dynamics, the company that makes the systems.

Thirty of the devices already have been installed in high-crime neighborhoods alongside video surveillance cameras.

The technology is gaining favor elsewhere, as well. The Los Angeles County sheriff's department plans to deploy 20 units in a pilot program, and officials in Tijuana, Mexico, recently bought 353 units, Baker said. Police in Philadelphia, San Francisco, New Orleans and Atlanta also have expressed interest.

The system deployed in Chicago — Smart Sensor Enabled Neural Threat Recognition and Identification, or SENTRI — uses four microphones to zero in on the shooter.

The city in 2004 reduced its homicide rate to its lowest level since 1965, and police seized 10,000 guns — successes that were in large part credited to a network of "pods," or remote-controlled cameras that can rotate 360 degrees and feed video directly to squad-car laptops.

The American Civil Liberties Union in Illinois is concerned about privacy rights being violated by the city's prevalent camera system. But as long as the cameras and SENTRI system are set up in public spaces, they do not violate the law, said Northwestern University Law School professor Robert Bennett. "You don't have much in the way of privacy issues when you're in a public area," he said.

And local officials say it's hard to argue with the results.

"The crime rates in Chicago are the lowest in 40 years. The price of keeping the community safe far outweighs civil-liberty issues," said Monique Bond, spokeswoman for the Chicago Office of Emergency Management.

Baker stresses that SENTRI is programmed to recognize only gunshots, not record conversations or "bug" private homes.

"There's no mechanism for other sounds like human voices," he said.

Each SENTRI contains a library of acoustical patterns, or "sound signatures."

They are used to differentiate gunshots from other noises, such as traffic and construction, by measuring the unique decibel level of a bullet being fired.

Adding SENTRI to an existing surveillance camera costs between $4,000 and $10,000 per unit.