Why does the sound of water change as bucket is filling from empty to full?

 I observed that when we start to fill an empty bucket with water, in the beginning

 we hear a loud sound of water touching the bottom of bucket . As the bucket gets

 filled with water, the sound becomes shriller gradually . 

Now when we have just the empty bucket, the whole space available inside the

 bucket is filled with air . Now we have turned on the tap and the water has started

 to fall . 

When water is getting filled, it occupies the space which was held by the air earlier.

 The previous air column which was made now slowly getting less and less . As

 sound requires medium to travel, and in this case it is air, the length of the air

 column is becoming shorter . 

So sound takes lesser time to vibrate, when the bucket is getting full.than the

 earlier . Lesser time for vibration means we hear the sound slowly . That's why

 when the bucket is empty, more time is available for the sound to vibrate, and we

 hear maximum sound. As the bucket is getting filled, lesser time for vibration and

 shriller is the sound .