I guess you could also use https://en.wikipedia.org/wiki/Sound_from_ultrasound https://en.wikipedia.org/wiki/Sound_from_ultrasound, which would mean unless you'

I guess you could also use https://en.wikipedia.org/wiki/Sound_from_ultrasound https://en.wikipedia.org/wiki/Sound_from_ultrasound, which would mean unless you're in it's line of sight you shouldn't hear it. I'm specifically thinking of the parametric array type. I'd love to see how directional that is sometime, as there are cheapish implementations of hardware for it. Edit: Originally I thought a human might be able to hear their sound, but reading a bit more I don't think that's the case since it's exploiting non-linearity of the microphone Also it seems ultrasound can be used to affect other MEMS devices: https://arstechnica.co.uk/gadgets/2017/07/sounds-bad-researchers-demonstrate-sonic-gun-threat-against-smart-devices/ https://arstechnica.co.uk/gadgets/2017/07/sounds-bad-researc... I wonder if there's any acoustic metamaterial that you could place over the mic etc. to filter ultrasound out before it reaches it.