Question

Suppose you have a simple pendulum. It sets the particles of air in vibration .Then why can't you hear any sound from it?

Answer 1

If something vibrates at a frequency of or in between 20Hz to 20KHz, we can hear it otherwise we can’t. because our ear can hear only this frequency range.

Most cases the oscillation of pendulum is less than the 20Hz(e.i. the swing of a pendulum isn’t grater than the 20 times in 1 second ).

that’s why we can’t hear the sound of pendulum oscillation.