> So given a 100 dB SPL transmitter (goodness knows how much electricity that will need… anyone want to install a three phase outlet by the bed?)

I don't know either, but I do know that you can buy a 110 dB ultrasonic transmitter [1] that is powered by 12 VDC @ 1A, so that sound level can be achieved with 12 watts (the power spec is on the back of the unit, which is visible in one of the photos). That's well below what will be sending anyone to the electrician to get three phase power in their bedroom.

As far as efficiency goes, there has been published work on this. I'm appending a comment I made in the discussion of this here 9 months ago that cited and discussed an interested research paper in that area.

[1] http://www.soundlazer.com/product/sl-01-open-source-parametr...

[2] https://en.wikipedia.org/wiki/Sound_from_ultrasound

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Any IEEE members here who could take a look at the following paper, which may shed some light on what is and is not possible here, and then summarize it for us?

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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

doi: 10.1109/IECON.2011.6119486

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.

URL: http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=611948...

Update: I was able to get a 5 minute preview of the paper from DeepDyve.com [1']. They got 53% efficiency at 1 meter, but that was at very low power (something like 37 uW). They weren't using particularly high powered transducers, though, so could have gotten more power if that had been what they were aiming for, but probably still a long way from charging a phone. They say that the challenge for acoustic energy transfer will be in developing the high power transducers that will be needed.

[1'] For those who have not heard of DeepDyve, they provide access to a very large number of journal articles for prices that are much lower than what the journals charge for individual articles. Yes, I know that ideally all of this stuff should be available to the public for free, but until that happens DeepDyve is worth considering. They have a "Freelancer" plan that costs nothing up front, and gives you 5 minute full previews of articles. You can purchase "tokens" in packs of 5 for $20. You can rent an article for 30 days for one token. They also have a $40/month subscription plan that lets you read unlimited articles, and gives a discount if you want to buy a PDF. You can cancel the monthly plan at any time, and your account converts to Freelancer, and you can switch back to the subscription plan at any time.