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Ultrasonic Power Transfer Investigated Using Data From uBeam Patent Filings (hackaday.com)
szczys writes: Transmitting power through the air using sound above the range of human hearing: that's the gist of ultrasonic power transfer. The promise is that you can sit in a coffee shop and use your phone like normal while it's recharged by invisible waves of energy. That's a future we all want — and one that uBeam has been promoting, but hasn't backed it up with proof. Physics is a cruel mistress, and this is no exception. Using the data found in uBeam's patent filings you can see that ultrasonic power transfer is a brutal engineering challenge. It's probably not impossible, but looking at what it would take for a widespread rollout of the tech makes it highly improbable.
Any technology which attempts to push power though a transmission medium where you have to worry about the "Inverse of the Square" of the distance is going to fail on it's face. Sound though air is such a problem. Magnetic and electric fields though air/vacuum is another. Power transfer may be possible, but the amount of losses means it will never be practical unless you have HUGE amounts of power to waste.
In this case you may not be able to hear ultra sonic frequencies, but that does NOT mean it cannot harm your hearing at the SPL's required to get any kind of power transferred.
"File to fit, pound to insert, paint to match" - Aircraft Maintenance 101
The whole planet is struggling to keep up with energy demands and you think it's acceptable to waste 90% of the energy you use?