There are two versions of the system. One uses a single microphone per module, but requires at least six separate phones to accurately calculate the location of the shooter. The second setup has four microphones in each (slightly larger) sensor module, but needs only two separate phones to locate the sniper.
The sensors make use of a distinctive feature of gunshots: they produce two bangs. One is from the muzzle of the gun, as the gases propelling the bullet expand and create a shock wave. That sound wave goes out in all directions, and it is shaped like a sphere.
Bullets make a second sound. As they are usually moving at supersonic speeds, it is a small sonic boom. The sound wave is conical, with the bullet at the point of the cone.
Timing when the two sounds arrive at the microphone can give a good idea of where the shot came from, and when that data is taken from separate locations, it allows for triangulating the sniper's position.
Using the sonic boom form the bullet allows the microphones to "hear" the shots over longer ranges. Janos Sallai, a research scientist on Ledeczi's team, told Discovery News that at larger distances, the spherical wave from the muzzle is distorted by obstacles on the ground. That's one reason people have a difficult time figuring out where sounds like gunshots are coming from.