Hmm, I see. I think that you can cheat quite a bit more than that, though, if your objective is only to fool human hearing (as your 48ksps and 20kHz numbers suggest): the humans can only use phase information to detect the directionality of sound up to a few hundred Hz, relying entirely on amplitude attenuation above that, presumably because their neurons run too slow. But maybe your objective is sonar beamforming or something.