Understanding in real time how we move, operate, and interact within living and working spaces is crucial to applications in smart buildings, elder care, and the automation industry. Myriads of systems have used complex electronics to capture this information, yet they often rely on independent sensors or devices that require active power and circuitry, also raising privacy concerns when using cameras or microphones. These solutions are often difficult to setup, have incompatible ecosystems, and require extra cost when upgrading already existing devices. In this paper, we propose SoundOff, a system that deploys ultra-low-cost, easily manufacturable, passive-ultrasound-emitting tags in any indoor environment (i.e., door knobs, toilet lids, cabinets, faucets, and windows), where the movement of this furniture causes a unique ultrasonic emission identifiable by a wearable device worn by users. These tags generate …