Question

Sound produced by vibration of simple pendulum is .......... sound.

Answer 1

Sound produced by vibration of simple pendulum is infrasonic sound.
Any vibrating body will produce sound unless it is in vacuum, in which case off course there would not be any sound, since sound requires a medium to propagate. When a pendulum swings from side to side it will obviously collide with the air particles in the air and they might vibrate a bit too producing some sound. But we don't hear it.
The vibrations do reach our ears, but because we can only hear from 20 Hz to 20,000 Hz, we don't hear it. The frequency of sound produced due to vibrations of seconds pendulum is 0.5 Hz which is infrasonic sound.