I've flown in an area with 15 or so of 4/6/8 rotor UAVs in relatively close proximity all about 250-350ft up and not only could you not hear them at all, you co

I've flown in an area with 15 or so of 4/6/8 rotor UAVs in relatively close proximity all about 250-350ft up and not only could you not hear them at all, you couldn't really see them either (the octorcopters looked vaguely like a bird). That was kind of the point of the event, we were all FPV flying and cooperatively calling out maneuvers to avoid hitting each other. (Edit: I'm obviously talking about it not hovering directly over your head at this altitude, in that case you will hear it. I'm talking about it passing over you at a fast horizontal speed at 300ft above you that you won't hear it unless your in near dead silent environment with no background white noise).