Question

Sound is produced when a body undergoes to-and-fro movement. However, no sound is heard when we wave our hand. Why?

Answer 1

The minimum frequency required to hear a sound by humans is 20 Hz. Which means 20 vibrations per second. By waving our hand, we cannot produce 20 vibrations in a single second. Therefore,the frequency of sound produced by waving of our hand is less than 20 Hz.
Hence, the sound produced by the waving of our hand cannot be heard as its frequency falls outside our hearing range.