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I wonder about this.

If you pour water in a closed tank, it makes a noise. I have noticed that as the container becomes more and more full, the pitch of this noise also increases. Why is that?

I do not study, nor have any knowledge of beyond basic concepts, of the physics involved in sound and vibrations.
However, my hypothesis is that this happens because the "chamber" where the sound is produced gets progressively smaller, therefore it behaves like a flute - as you change the shape of the chamber, the sound that air makes as it passes also changes.

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  • $\begingroup$ Hot chocolate effect? $\endgroup$
    – user43617
    Nov 4, 2014 at 17:00
  • $\begingroup$ @Jun-GooKwak: apparently, not. The hot chocolate effect involves the pitch change "after the addition of a soluble powder", according to Wikipedia; this is not the case. The phenomenon I described is, for example, what happens when you fill a bottle. $\endgroup$
    – Giulio Muscarello
    Nov 4, 2014 at 17:02
  • $\begingroup$ @GiulioMuscarello See physics.stackexchange.com/questions/92318/… $\endgroup$
    – user43617
    Nov 4, 2014 at 17:04
  • $\begingroup$ @Jun-GooKwak Thanks, looks like it's an exact duplicate. I'll close this question. $\endgroup$
    – Giulio Muscarello
    Nov 4, 2014 at 17:28

1 Answer 1

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I think you've got it right. The air space above the liquid creates a chamber that supports resonance at specific frequencies, similar to a flute. Since the chamber may be oddly shaped, it will in general support multiple frequencies or a range of frequencies (unlike a flute, which is designed to resonate at only one frequency). However, since at least some of the dimensions of the air chamber in your water vessel get smaller as you fill it, at least some of the resonant frequencies rise, and you hear the pitch go up.

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