A portable radio is sitting at the edge of a balcony 5.1m above the ground. The unit is emitting a sound uniformly in all directions. By accident, it falls from rest off the balcony and continues to play all the way down. A gardener is working in a flower bed directly below the falling unit. From the instant the unit begins to fall, how much time is required for the sound intensity level heard by the gardener to increase by 10 dB?