When you pour a glass of water, why does the pitch of the sound of the water going into the glass get higher?


When you pour a glass of water, why does the pitch of the sound of the water going into the glass get higher?

In: Physics

Sound is made of waves. You can picture it like if you tie an end of a rope to a wall, and hold the other, waving it up and down. The faster you do that, the higher the frequency (which means the pitch is higher. No matter what, the side that’s tied to the wall is a node (a point that doesn’t move) and the side you’re waving is the antinode (a point which moves the full distance from top to bottom). The same happens in a tube, like a glass of water. Now, whichever frequency takes the least energy and fits like that will be the pitch you hear. As you fill up the glass, it’s like you’re holding the rope at a point closer and closer to the wall, so the frequency has to be higher, and that’s why the pitch you hear changes.

The sound bounces around in the air at the top of the glass and the dominant frequency is directly influenced by the size of the space. Larger spaces can allow for larger wavelengths (i.e. lower frequencies). So as you decrease the volume of air by filling the glass, the pitch goes up.

You know how different size drums make different notes, right? A little toy drum makes a high pitch *pang-pang*. A regular drum kit makes lower sounds, and the biggest drum in that kit (the bass drum on the floor) is the lowest. Then there’s the [huge Japanese drums](https://i.ytimg.com/vi/C7HL5wYqAbU/hqdefault.jpg) – if you’ve ever heard them, they’re seriously low and booming.

Well, the air in a glass vibrates just like the air in a drum. The fuller the glass, the smaller the air-space, the higher-pitched the “drum”.

(For one more level of detail “*why* is a smaller drum higher pitch?” Well “pitch” is the same as “frequency” aka how many times the air vibrates per second. Sound moves at a given speed, so the smaller the space it’s trapped in the faster it will be bounced back and forth between the walls aka higher frequency/pitch.)

The sound you hear is the longest wavelength the sound can travel in the glass, jar, bottle or whatever you are filling up. The longer the distance, lower the pitch and vice versa. When you fill up a bottle, the distance the sound can travel in the bottle gets noticeably shorter as you fill it up.

If you have a container that is cylindricar in opposite way to a bottle, so that the top is wider than or as wide as the bottom, this effect doesn’t happen as noticeably because the longest distance stays the same. It still happens, but it’s less noticeable.