Surveillance cameras in the city of Groningen have been adapted to listen out for voices raised in anger. Microphones attached to the cameras feed the sound signals to software that can detect voices that are aggressive in tone. "Aggressive people tend to tense their larynx, and the sound made by their vocal cords is distorted," says Peter van Hengel of developer Sound Intelligence, a spin-off of the University of Groningen.
In a trial earlier this year, police made three arrests after being alerted by the system. Tests are also under way in Rotterdam, on Dutch trains and in stations.
Sound telescopes from microphone arrays
Mundane spy gear