Nobody has yet mentioned, perhaps because it's obvious: every time you change the density of the medium in the path of the sound beam, you get reflection or refraction losses.
Hands are much denser than air. Not only does UBeam need to track where the receiver is, they need to hit the target without going through your hand or body to avoid another large efficiency loss -- I believe the typical figure for air/soft tissue boundaries is 99%.
Hands are much denser than air. Not only does UBeam need to track where the receiver is, they need to hit the target without going through your hand or body to avoid another large efficiency loss -- I believe the typical figure for air/soft tissue boundaries is 99%.