When we start filling an empty bucket with water, the pitch of sound produced goes on changing. Why?

Explanation

A bucket can be treated as a pipe closed at one end. The frequency of the note produced `=upsilon//4L` , where `upsilon` is velocity of sound in air and L is length of air column, which is equal to depth of water level from the open end. As the bucket is filled with water, L decreases. Therefore, frequency of sound produced goes on increasing.