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by dijksterhuis·6y ago·view on hn ↗
Filter won’t necessarily help. It’s possible to use ultra sonic for amplitude modulation signal to affect the non linearity of a microphone itself. [0]

Although it does require knowing what the underlying natural sound to be modulated is (some processing required).

Robustness to background noise doesn’t necessarily help either.

0: https://arxiv.org/pdf/1708.09537.pdf