why does the more water I pour into a container, the higher the pitch of the sound goes?
In: 9
When you pluck a guitar string, it will vibrate with a ton of random frequencies at first. But most of those frequencies won’t “line up” with the length of the guitar string and will almost instantaneously die out. Only the ones that do match the length of the string will keep vibrating, sapping all the energy from the bad vibrations and causing them to grow. This is called *resonance*.
Air in a pipe that is open at one end can do a similar thing. It’s a little hard to picture, as the thing that’s “waving” isn’t visible, but it can resonate just like a guitar string can. This is the principle behind how most “blow-into-it” instruments work. In particular the flute, which is literally just a pipe with a hole in the end.
The holes in a flute that a flute player covers and uncovers with their fingers (or presses buttons that open valves which do the same thing) are there to make the flute behave like it is shorter than it really is. The resonance only works as far as the tube is sealed up, as soon as it gets to the open end that’s more or less the end of the pipe. A simple hole in the side of the pipe is enough to disrupt that. So by covering all the holes, you get a pipe that is as long as the entire flute, but with some of the holes open, you’re essentially playing a one-note flute that’s only as long up to the first uncovered hole. (There’s probably some subtle resonance and overtone stuff that makes the reality much more fiendishly complicated than this, but that’s the gist of it.) So by using your fingers to rhythmically plug and unplug holes along the flute, you can play your one flute like it was a lot of one-note flutes of different lengths, all tuned to different resonant frequencies. Or in other words, you can play multiple notes on it.
Plugging and unplugging holes is just one way to go about changing the effective length of your pipe. If you don’t care about clean steps between note pitches, you can just put a sliding plug inside the pipe that you can slide in and out. So when you slide the plug back and forth, the length of your pipe goes up and down smoothly, and the resonant frequencies of the pipe smoothly slide up and down to match. This is the operating principle behind a [slide whistle](https://www.youtube.com/watch?v=0M2OgPeMO8E).
Knowing all of that, we can now analyze your glass. What is a drinking glass but a very wide, stumpy pipe that’s open at one end? And we know that if you cause the air in an open pipe to vibrate (perhaps by, say, splashing water into it), only the frequencies that match the resonant frequencies of the pipe will survive and cause it to ring. And if the length of that pipe changes (perhaps by, say, the surface of a liquid being poured into it slowly climbing up through it), the effective length of the pipe changes, causing the frequency to smoothly change with it.
tl;dr by filling a glass with water, you’ve essentially created a really shitty slide whistle.
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