[huron]

This is the same principle as DEX. Just with a speaker / mic setup instead of some sort of rs232 cable with 1/4" jack. What would be interesting is if you could use a sonic range below what humans can hear and still be effective with an open speaker / mic setup? Lights, hvac type controls would be very doable.

[Sanddancer]

Not really. The human ear can perceive down to 20Hz, and below that, you can't perceive it actively, but the subconscious starts playing weird tricks [1]. Plus, you'd be limited to X Hz/2 bits per second. Your better bet is getting something to drive an occasional chirp at 20k hz or so, which would be imperceptible to most people.

[1] http://skeptophilia.blogspot.com/2011/02/ghost-in-machine.ht...