It just occurred to me that the / (4 pi r^2) factor may not be relevant here. The reason is that the acoustic wave could be steered (http://en.wikipedia.org/wi

It just occurred to me that the / (4 pi r^2) factor may not be relevant here. The reason is that the acoustic wave could be steered (http://en.wikipedia.org/wiki/Beam_steering http://en.wikipedia.org/wiki/Beam_steering) so it does not propagate evenly over all the solid angle. Perhaps the steering could be done cooperatively with the device(s) being charged. I.e., devices announce their location to the charger, and the charger steers the beam towards them. Maybe that would bring the power required down to a reasonable range.