I'm not saying we should be building ultrasonic wearables that emit a string of wake words and nefarious voice prompts that can only be picked up by nearby smart devices, but that is totally within the realm of possibility.
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If you go far enough back, you'll find a project where Naomi Wu (@SexyCyborg) built a choker necklace out of ultrasonic transducers that jams nearby embedded microphones with white noise (external pro-grade mics unaffected).
https://youtu.be/H1rozZ7ebxQ
Based on this project: https://sandlab.cs.uchicago.edu/jammer/