Aren’t there theoretical methods for columnarly focusing sound waves? I’m not saying uBeam has that. Just that there’s a middle ground between “it’s impossible” and “uBeam pitched crap.”
Sure you can focus sound waves. But they're still subject to the inverse square law. And the tightness of the beam is proportional to the size of the emitter: you need a huge array to maintain focus over any reasonable distance. Plus there's no theoretical way to entirely get around the efficiency or safety concerns.
So not literally impossible, but totally impractical as a commercial product.
This is a broader case of all wave phenomena. As a VERY coarse guide, the equation is :
R = Lambda/D
Where R is the angular 'resolution' of the device, Lambda is the wavelength of the wave used, and D is the diameter of the device.
This means that as the 'resolution' gets smaller (ie. better), the diameter becomes bigger. For example, if you have a 20kHz sound source (~1.7 cm wavelength) and want an angular resolution of ~ 1 arc-second, then you'd need a ~3.5km diameter array. Actual equations will vary GREATLY from this, but it's a good 'sniff test' starting guide.
