The problem is that any idiot can get 5 volts out of thin air. You can literally hang up a long bare wire and rectify ambient radio waves and get 5 volts. The trick is getting 5 volts and being able to supply significant amounts of current at the same time i.e. receiving useful amounts of _power_. Power = Volts x Amps and Meredith and her investors simply did not understand the importance of this equation that a freshman in engineering would understand. FOMO kicked in and they threw millions of dollars at her thinking it was going to be the next big thing.
I don't even think that as a freshman, I would have gotten it. On the other hand, I was pretty loosy on the EM side of things, that's why I then went into software quickly afterward .
No, the air isn't holding a charge in Meredith's setup. The setup is essentially a speaker and a microphone, but optimized (somewhat) for energy transmission.
It's actually possible to transmit sound waves without the exponential decay with "directional sound" [1]. It is produced using a "phased array" [2] of speakers, where each speaker has a different a phase. The point is to create constructive interference in the direction you want the sound beam. Directional speakers have existed for a while and that's probably how uBeam emitters work.
2. Ultrasound transducers are very inefficient, so you’ll lose a lot of power.
3. Ultrasound transmission will be blocked by a person, or even by a sheet of paper in the way.
4. Even if you do achieve a tight beam, then you either have to actively steer it around, or it’s only good for transmitting power to something sitting still in a specific spot.
5. We don’t really know much about the effects of having watts of ultrasound present in a space with humans 24/7/365.
(And these are just the easy ones)
Roes, M.G.L.; Hendrix, M.A.M.; Duarte, J.L., "Contactless energy transfer through air by means of ultrasound," IECON 2011 - 37th Annual Conference on IEEE Industrial Electronics Society , vol., no., pp.1238,1243, 7-10 Nov. 2011
Abstract: An alternative approach to the wireless transfer of energy is proposed, employing acoustic waves in air. Unlike conventional methods, acoustic energy transfer is able to achieve energy transfer at high efficiencies over distances that are large in comparison to the dimensions of the transmitter and the receiver. This paper gives an overview of the principle and explains the different loss mechanisms that come into play. A theoretically limit on the achievable efficiency is calculated. It exceeds that of a comparable inductively coupled system by an order of magnitude. First preliminary measurements indicate that AET is feasible, although the measured efficiency is lower than the predicted theoretical limit.
They were using small low-power transceivers as their focus (no pun intended) was on large distances compared to transceiver size, and so were only dealing with a few dozen uW power.
Looking at a few of the things that cite them, it seems that low power is where most of the work in ultrasonic power transmission is, aiming for environments where you need to power something small in a location that makes the usual methods problematic, such as implanted biomedical devices. Example: "MEMS Based Broadband Piezoelectric Ultrasonic Energy Harvester (PUEH) for Enabling Self-Powered Implantable Biomedical Devices", https://www.nature.com/articles/srep24946