If you read the FAQ on eevblog, 100 KHz frequency ultrasound will lose 90% of its energy at 2 meter range or just over 6 feet. That energy goes to heating the air that the sound travels through.

Then you have to contend with any orientation offset. If the receiver is off by 90 degrees or more, there is zero energy received. If your hand blocks half the surface area, that's another 50% power loss.

If you boost the power above 140 dB safety limit, you can burn your hands or cause other problems. At 140 dB at 2 meters, the theoretical best case with 100% efficiency transmitter and receiver will deliver 0.85 watts. You obviously lose even more than that.

http://www.eevblog.com/forum/projects/the-ubeam-faq/