Absorption in Air and Wireless Charging

Apr. 2018

In a previous blog post we discussed diffraction, a phenomenon that prevents beams from being focused into arbitrarily small spots.

Another factor that might limit distance is air absorption. The energy levels of different technologies decrease by different amounts when travelling through air. Absorption particularly impacts ultrasound technologies. The exact decrease depends on the temperature and humidity, but approximately 50% of the ultrasound energy is absorbed (and thus lost) in air for every 1 meter (~3 ft) traveled.

Combining diffraction and absorption, here are the approximate levels of energy that can be captured by a 5 cm receiver after travelling through a certain distance.

In contrast to IR light, both RF and ultrasound suffer substantial energy losses over distance, whether because of diffraction or absorption. RF and ultrasound need to transmit at very high power to deliver useful energy levels. But are these high power levels safe? We’ll cover that in a future post.

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