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Here's how uBeam actually works:

Ubeam emits Ultrasound frequency waves from a base station. These sound waves are converted into mechanical energy by a crystal inside a flash-drive sized dongle attached to the device. The crystal resonates at that sound frequency and the mechanical energy of the vibration is then converted into electricity by the dongle.

The shit works. And everyone told her that it couldn't.

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50kHz ultrasound in the demo.

I expect the efficiency to be terrible, which will be the failure point. Do you want to have an always-on 300W device just to avoid finding a cord?

Radial power transfer efficiency drops off proportional to r^2, as does omni-directional sound. Unless there is some sort of directional beam established to focus the energy transfer, I imagine these power losses will indeed occur. Note that room effects may well be non-negligible - reflections may mean more power arrives at the receiver than the naive estimate.

So I would imagine the ceiling mounted charging station would be a fairly heavy duty outfit with a multi-kW rating if it is to be able to service a large room.

I thought prolonger exposure to ultrasound was dangerous, how is uBeam safe?

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