The shape of the metal tag — a small disk with a hole in the center like a flat washer and various cutouts along the outer edge — determines the frequency of the sound, so each tag can be uniquely identified.

“Those unique fingerprints can be used for smart home sensing, or what we call ‘activity recognition,’” said Yibo Fu, a robotics Ph.D. student who led development of the tags with other engineering and computing researchers. 

Fu said the tags could be attached to faucets to help monitor water use or toilet lids to alert caregivers that an elderly relative might need assistance in the bathroom. Attached to weights in the gym, they could count squats or presses. Users could manually press button versions to trigger a timer or log an activity. 

Fu recently described the tags on Instagram in a viral video that’s generated 1.6 million views and counting, 150,000 likes, and hundreds of comments. Among the reactions are people proposing other uses and asking for a crowdfunding campaign to make them a reality.

“There are some pretty interesting comments from people in other fields,” Fu said. “One mentioned using the tags in archiving systems where you have huge shelves and libraries of boxes. When you remove a box or store a new box, there's a rapid motion, and you would trigger the tags and know exactly what thing you just opened, closed, or archived. Someone else mentioned tracking locations for thousands of garbage and recycling bins in waste management systems.

“Those are more specialized scenarios, but it’s been fun to see those comments and ideas.”

Bolei Deng in the Daniel Guggenheim School of Aerospace Engineering specializes in vibration and waves and how geometry of an object influences its resonance. He and his team created a modeling and simulation tool to design the metal disks so they would generate specific ultrasonic frequencies when they’re struck.

Their simulations identified nearly 1,300 initial designs that would each produce a unique frequency in the ultrasound range. Those frequencies are above 20 kilohertz, which is the upper limit of sounds humans can hear. The team used 15 of the proposed designs in their tests.