From what I understand, uBeam's low power demo `works` because they're still (mostly) in the linear range. As you scale it up, you run into a couple of nonlinea

From what I understand, uBeam's low power demo `works` because they're still (mostly) in the linear range. As you scale it up, you run into a couple of nonlinearities that form an upper limit of the power density and distance. 1) You can't run sound at over 1 atm of overpressure, because the troughs can't get to negative pressure. This limits the amount of power that you can physically get into a region using sound pressure. 2) Dispersion is a nonlinear effect that changes the frequency distribution of the sound, which has the effect of reducing the focus effect at greater range. (Also, affects the absorption losses). 3) Even in fully linear systems, phased arrays don't focus all the energy into one area, they have significant side lobes. You can monkey with the shapes and intensities, but TNSTAAFL. The SPL that they're talking about is in the range of jet engines at close range. Yeah, it's ultrasonic, and yeah, it's focused. But I don't want to get anywhere near the thing. The world is nicely linear, until it's not. You have to know where the simplifying assumptions end.