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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 ultrasound signals during everyday interaction, above the range of human hearing, making them non-intrusive and non-invasive. This electronics-free, zero-infrastructure solution provides a scalable way to instrument spaces. Through a series of performance evaluations, we show that the ultrasonic emissions generated by SoundOff with different geometrical designs are not only robust to various environmental noises but also easily distinguishable from each other. Through physics-based modeling, we demonstrate how to systematically generate thousands of designs with unique and easily distinguishable ultrasonic emissions, enabling a wide range of interactions that can be mapped to custom automation systurce the work, including a geometric modeling pipeline and fabrication guide that enables design exploration, as well as an easily modifiable recognition system, allowing SoundOff tags to be replicated and disseminated throughout any indoor environment. 

In this Letter, we investigate the propagation of nonlinear pulses along the free surface of flexible metamaterials based on the rotating squares mechanism. While these metamaterials have previously been shown to support the propagation of elastic vector solitons through their bulk, here, we demonstrate that they can also support the stable propagation of nonlinear pulses along their free surface. Furthermore, we show that the stability of these surface pulses is higher when they minimally interact with the linear dispersive surface modes. Finally, we provide guidelines to select geometries that minimize these interactions. 

Abstract Multi-welled energy landscapes arising in shells with nonzero Gaussian curvature typically fade away as their thickness becomes larger because of the increased bending energy required for inversion. Motivated by this limitation, we propose a strategy to realize doubly curved shells that are bistable for any thickness. We then study the nonlinear dynamic response of one-dimensional (1D) arrays of our universally bistable shells when coupled by compressible fluid cavities. We find that the system supports the propagation of bidirectional transition waves whose characteristics can be tuned by varying both geometric parameters as well as the amount of energy supplied to initiate the waves. However, since our bistable shells have equal energy minima, the distance traveled by such waves is limited by dissipation. To overcome this limitation, we identify a strategy to realize thick bistable shells with tunable energy landscape and show that their strategic placement within the 1D array can extend the propagation distance of the supported bidirectional transition waves.