Bolted joints fail quietly, long before anything visible happens. This sensor bonds to structural steel, listens for the acoustic signature of a loosening bolt, and transmits an alert. No access to the bolt head, no wired infrastructure, over a year on a single battery, under $25 in hardware. Bridges, wind turbines, and industrial facilities have thousands of these connections and no way to monitor them continuously. The physics behind the detection method is decades old and well established. A permanently installed, wireless, commodity-hardware implementation is the part that has not existed.