How it works: Sensitive microphones inside a palm-sized external sensor box pick up the sound and atmospheric wake from a bullet. Then the sensor box, which includes a precise clock, wirelessly relays the data to a smartphone. It takes six of these external units to obtain accurate location information (which is no problem for a squad of troops or a police team.) Another version of the app, created by the same researchers, uses two larger sensor boxes and only picks up on the shockwave. It is less precise, but still far more accurate than human hearing. The technology was originally developed for soldiers' helmets.