Have you ever noticed that when you are filling a container with fluid. As it approaches the top, it makes a different sound? You can tell by listening when your about to reach the top. Why is this?
[Physics] Why does a container with fluid make different sounds at different fluid levels
acousticsfluid dynamicsresonance
Best Answer
It is caused by standing waves in the container. You get, as a result, harmonics. There are overtones occurring for a fixed frequency.
The changing sound is because a water filled container is like the half open model in the picture below. As the water level rises, the length of the tube decreases. This would lead to a change in the frequency of standing waves in the tube, thereby leading to different sounds.
The wavelength of the standing wave is a function of $L$.
$$ v= f\lambda \implies f = \frac{v}{\lambda} \implies f = \frac{v}{4L} $$