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

Explanation

We can consider a bucket to be a closed organ pipe. So, it produces note of frequency.
`v = (upsilon)/(4L)`
where `upsilon` is the speed of sound in air and L is the length of air column, which is equal to the depth of water level from the open end. As the bucket starts filling with more and more water, the value of L decreases. As a result, the frequency or pitch of the sound keeps on increasing.