To a soldier in a foreign, hostile city, the crack of sniper fire can be as confusing as it is terrifying. The walls and corners of buildings and tunnel-like city streets can reflect, bend, and channel sound waves, making it impossible to identify their point of origin—for a person, at least. An experimental new computer program can track echoing sound right back to its source.
Lanbo Liu and Don Alberts, geophysicists with the U.S. Army’s Cold Regions Research and Engineering Laboratory in Hanover, New Hampshire, placed a series of microphones throughout a full-scale military training village containing one- and two-story concrete buildings. When a sniper fired a shot into the city, the sensors recorded the sounds and relayed the information to a computer containing a digital replica of the city. The computer then created a time-reversed version of the sound waves that struck each sensor, zeroing in on the place where they all converged: the location of the sniper.
The system should work in any urban environment that can be outfitted with sound detectors and premapped into a computer. In fact, complex urban areas (central Baghdad or New York City, for instance) generate more sound paths and therefore could yield more accurate results. Moving objects, such as cars, complicate the analysis, however. “Currently, it takes hours to process the data. We’ll need faster algorithms and bigger computers to do it in real time,” Liu says. He expects to have a field-ready model within a few years.