In areas such as North Lawndale, where customers were once scarce at the dollar stores, African hair-weaving salons and second-hand car dealers, it is already making a difference.

Gang members and drug dealers once fired their weapons indiscriminately with little fear of reprisal. The community was paralysed.Emma Mitts, the alderman in charge of the borough, said: “Before they had the gun detection, let me tell you, it was rough. They were shooting at each other in broad daylight, round schools — gangs getting into gang wars.

“With these sensors now, at least they know it’s not gonna be easy for them to go out there and shoot open market.” Chicago is the first city to install Sentri (Smart Sensor Enabled Neural Threat Recognition and Identification).

The cameras can detect a gunshot within a 350m radius and instantly zoom in on the source and the culprit. The image and co-ordinates are sent to a command centre and police move in.

Ron Huberman, executive director of Chicago’s emergency management and communications office, says that surveillance helped to reduce murders in Chicago to 447 last year — the lowest since 1965.

Cameras alone were not enough, he said. “What this technology does is it makes it incredibly difficult to fire a gun because if you do, we’re on top of you in a second. We are seeing the tangible results of that. There’s a dramatic decrease in firearms-related crimes.”

Chicago has mounted five Sentris and will add 80 by the end of this year. Los Angeles has begun trials and police in San Francisco, Phoenix, Oklahoma City, Baltimore, Philadelphia and Atlanta have inquired about the system.

The Sentris cost $32,000 (£17,000) each and are being funded from assets seized from convicted drug dealers. In Los Angeles, television networks are bidding for the right to foot the bill for the Sentris. The winner will get exclusives crime stories and the CCTV tape of the the incidents. The sensory technology was designed by Theodore Berger, director for neural engineering at the University of Southern California and co-founder of Safety Dynamics, which builds the Sentris. He based it on the human hippocampus, the part of the brain responsible for memory, which forms patterns for every new sound and reviews memories of noises to recognise them.

Dr Berger played the computer countless variations of 45-calibre gunfire until it formed a concept of a 45. He used the same technique to teach it to disregard sounds such as a car backfiring.

His work is funded by the Office of Naval Research in Arlington, Virginia, which is testing a program called Gun Slinger in the California desert. This involves a Humvee allterrain vehicle fitted with a small “listening” mast; if it detects gunfire, vehicle-mounted machineguns swivel in the direction of the sound to help soldiers to aim.

Dr Berger is working on similar technology that could overhear and recognise words such as “explosives” or “Kalashnikov”, in a noisy environment and even in a foreign language.

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