Four microphones in the system differentiate gunshots from other noises like traffic and construction by measuring the unique decibel level of a bullet being shot out of a gun, and comparing the sound to its library. That way, a gunshot would activate the system, but a siren or a car backfiring would not, Baker said.
"We take recordings of the target sound and you take as many different recordings as you can from as many different types of scenarios as you can," he said. "But it's a little bit like fingerprinting. The more precise the sound we get, the greater the possibility that we eliminate some potential gunshots."
Adding the SENTRI to an existing surveillance camera is not cheap. The system costs between $4,000 and $10,000 per unit, but in Chicago they and the accompanying cameras are paid for with forfeiture money.
Police Superintendent Phil Cline told an audience at a recent U.S. Conference of Mayors meeting, "the drug dealers are actually paying to surveil themselves."