Re: Have amazon actually made this?
Completely agree with you, fairly basic physics any A Level student would (should?) understand, the noise created by rotating propellers can be (mostly) cancelled by another propeller.
Unfortunately the secondary propeller needs to be continuously pushing back against the shockwave for the full rotation, meaning that it rotates with the primary propeller and more importantly AoA reversed so it effectively pushes in the OPPOSITE direction to the primary propeller!
Contra-rotating will actually create a beat in addition to the noise of two propellers!
Who are they employing to come up with these ideas I wonder, becuase its obviously not engineers!