It sounds simple: They devised a set of penny-sized, cheap metal tags with unique ultrasonic fingerprints that can be used to detect the opening of a door or window, and other similar modest movements. These tags, dubbed SoundOff tags, are totally passive, requiring no battery or backscatter scheme for power. At the same time, the algorithm that recognizes and identifies each unique ultrasonic signature is fairly straightforward and doesn’t need a lot of computing power. When struck, the tab creates an ultrasonic “ping,” which is distinct to that tab due to its edge cutouts. A microphone and signal path optimized for ultrasonic signals picks up the sound, which is then analyzed via digital signal processing to identify which tab was hit (Fig.