AcousTag : Batteryless 3D-Printed Acoustic Tag for Smart Speaker-based Event Monitoring

Monitoring everyday events in the home enables computational systems to provide timely support without imposing continuous user effort. However, existing approaches present key trade-offs. Cameras raise privacy concerns, while wearables require continuous use and charging. Tag-based systems offer a more practical alternative, but either need periodic battery replacement, rely on non-pervasive transceivers, or demand multiple receivers per room due to limited sensing range. We present AcousTag , a batteryless , 3D-printed tag sensed by pervasive smart speakers (e.g., Amazon Echo Dots) that detects interactions with hinged or sliding objects at a distance. AcousTag introduces two key technical components: a batteryless tag hardware design that harvests motion energy to generate unique, identifiable acoustic reflections, and a receiver-side software design that retrofits commodity smart speakers to detect these reflections in real time, enabling room-scale detection and differentiation of multiple object interactions. We evaluate AcousTag in real-world settings by deploying tags across two homes, achieving average activation accuracy of over 94%. We further demonstrate that tags remain detectable at distances of up to 11 m, while maintaining robustness to multi-speaker operation and simultaneous tag activations.