Question

Fill one bottle about one-fourth of the way full with water. Gently tap the top of the bottle with a metal utensil or some other solid object. Gradually fill the bottle with more water, and keep testing the sound it makes when you strike it. Note how the sound changes as you increase the amount of fluid in the bottle. Repeat the steps above but this time blow gently across the top of the bottle to create a sound.

1. Why is there a difference between these two noises? In your answer, comment on the source of the noise in each case (i.e. What is vibrating?).

Answer 1

When we make a sound above an open bottle, the sound vibrates inside the air column in the bottle. When we fill water in the bottle , the height of the air column decreases.

The frequency of the sound vibrating in the air column depends on the height of the column. As the height of air column decreases, frequency and pitch increases.

So in both cases, pitch of the sound increases when more water is filled. This is why there is a difference in sound heard both cases.