Drones can vanish by humming in a city's background noise

A drone flying over a quiet meadow is nearly impossible to miss, but the same machine can become a ghost in a crowded city. Small multirotor drones produce a signature buzz because their propeller blades slice through the air at specific intervals. For example, a four-bladed propeller spinning at 6,000 revolutions per minute creates a distinct tone at 400 hertz. Because the human ear is most sensitive to frequencies between 2,000 and 5,000 hertz, designers are now selecting motor and blade combinations that shift this sound energy into ranges where our hearing is naturally dull.