Answer:
One cannot hear sound when moving hands in air because moving hands in air is not considered as vibration. This is because the movement is very slow.
The slowest vibration that human ear an detect is 20 times a second. Then it can be considered as vibration as the movement of hand in air is less than this speed so it is detected by ear.
The fastest vibration that can be heard by ear is 20,000 time per second. So, the answer is moving hands in air is not vibration so it is not heard by ear.
