Comment by timdierks | Hacker News Reader

Comment by timdierks | Hacker News Reader

I don't know a lot about specific engineering details, but a jet engine (turbofan) produces about 1 kw of sound energy [1]. A cell phone has about 75 sq. cm of area on one side. Thus, with perfect efficiency and spherical propagation, the available power to the phone for a user about 3m away would be about 0.2 watt.

However, I'm skeptical that they can actually create a kilowatt of sound energy, and while you can focus the power (certainly you could do something more efficient than fully-spherical propagation), you're also going to have significant losses.

All this for a charging rate that's about 1/5 of what you get from a 1 amp USB charger.

[1] https://en.wikipedia.org/wiki/Acoustic_power#Table_of_select...

Unless they are beamforming, which is highly unprobable, I think you're right.

You can do beamforming with ultrasound. http://www.ted.com/talks/woody_norris_invents_amazing_things The demo device was flat, like the one shown in the article.

Beamforming would be cool and might actually allow this to work, although I don't know with what efficiency.

That said, the tech just to locate the phones in space to allow the beamforming would be a significant accomplishment itself. But forming the beam or multiple beams to charge multiple devices is probably quite feasible.

A possibly effective option would be sweeping the beam around and having the phone report when it hits. That should be easier than trying to accurately locate the phone first so that you can aim a beam at it.

If you can also make a reasonable initial guess of it's location (to within a few degrees) you could narrow down the guess and check search area considerably.

http://en.wikipedia.org/wiki/Long_Range_Acoustic_Device

http://science.howstuffworks.com/lrad.htm/printable

The NPR article[1] gives some more information on what it is. Apparently, it's ultrasonic piezoelectric charging. My guess is that it's using a resonance frequency approach to vibrate the crystals.

1: http://www.npr.org/2011/08/23/139854129/young-entrepreneur-h...

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