I believe the original comment isn’t about the receiver, but about the emitter—that if you use audible-range sound or even ultrasound, the spatial resolution of the signal will be impossibly large compared to a nanobot. Each nanobot will be able to get the signal, but you won’t be able to only communicate with nanobots in a specific part of the body.
This might not be a fatal objection, since you could imagine some sort of protocol with unique addresses or whatnot, but it’s an objection.
I believe the original comment isn’t about the receiver, but about the emitter—that if you use audible-range sound or even ultrasound, the spatial resolution of the signal will be impossibly large compared to a nanobot. Each nanobot will be able to get the signal, but you won’t be able to only communicate with nanobots in a specific part of the body.
This might not be a fatal objection, since you could imagine some sort of protocol with unique addresses or whatnot, but it’s an objection.
This isn’t about bots, it’s about a little tiny factory building your second-stage materials.