ultrasonic transmitter for headphones

When considering constructing a `wireless` headphones system, three basic approaches present themselves: a `genuine` r.f. transmitter and receiver; an infra-red system em-ploying I R LEDs and photodiodes, or an ultrasonic system. Since the first method is illegal and the second is both relatively complicated and expensive, we are left with ultra-sonics, if we are looking for a reason-ably simple and cheap transmitter/re-ceiver.