A vibrating tuning fork is placed over the mouth of a burette filled with water. The tap of the burette is opened and the water level gradually starts falling. It is found that the sound from the tuning fork becomes very loud for a particular length of the water column.
Why does the sound become very loud for this lenght of the water column?

Explanation

The frequency of the tuning fork and the natural frequency of the vibrating air column become equal. The air column vibrates with larger amplitude thus producing a loud sound.