Question

When a tuning fork (vibrating) is held close to ear, one hears a faint hum. The same [vibrating tuning fork] is held such that its stem is in contact with the table surface, then one hears a loud sound. Explain.

Answer 1

When the vibrating tuning fork is held near the ear, only a small amount of air is set into vibration, so a faint sound is heard.

When its stem touches the table, the table is forced to vibrate with the tuning fork. Since the table has a much larger vibrating surface, it sets more air into vibration and produces a louder sound.