Why does the sound of filling up a water bottle change the higher up it gets?
In: Physics
Because as the bottle fills the space the sound is bouncing around changes. When you’re filling up an empty bottle the sound is coming from relatively far away, as opposed to the top where it’s moving a small distance and thus has a different frequency (sound). It’s similar to putting your finger on a closer to further hold on a wind instrument, but with water.
The sound we hear travels from the source of sound to our ears in the form of waves. During this process, the wave hit and bounce back on the nearby surfaces before reaching our eyes. As the bottle gets filled, the sound waves have lesser solid space to hit and bounce back. Some waves are absorbed by the water. Sound waves travel differently through water than when hitting a solid. This is why the sound changes as the bottle fills up.
A side note. Let the bottle overflow and listen to the sound and you will just hear only the water dripping and not the *shurrrrrr* sound because soundwaves no longer hit the inner sides of the bottle.
The first thing to know is air does not carry all sounds equally. Depending on the temperature and pressure and the shape of the container in which the air is, some sounds get carried better than others. In this case, the sound is generated where the falling drops hit the surface of the water. The sound is carried outside the bottle by the air above the water. You can think of the generated sound as being made up of multiple smaller sounds(of different wavelengths). The shape(length) of the air above the water surface is changing as we fill water. This causes sounds of different types(wavelengths) to carry differently.
Just ignore the part about wavelengths and it should be an ELI5. Look up standing waves if you want to read further about this.
I think I will piggy back this question with another similar one:
Why does hot water sound different than cold water? If you ever noticed:
Cold water sounds more metallic and tingy, while hot water sounds more silky and smooth when poured.
Water running produces noise, which is basically a bunch of different tones at once.
Now, a tone can be represented by an arc, the length of two of them being the length of the wave (a wavelength). The wave is thus made of a knot and hill.
Imagine the wave starting inside the bottle with a knot at the water’s surface. If a wavelength exits the container at a length where it exits as a “hill” then it becomes amplified.
As the height of the water changes the wavelengths that get amplified change as well. That produces the unique sound.
Of course there’s also a lot more at play, but that’s the main fist of it without diving into acoustics.
Resonance.
Sound is a vibration in air. Air can be compressed and will bounce back, it’s “springy”. That compression and return is a vibration, which we hear as sound.
The smaller the volume of air, the more springy it is, the faster it bounces back and forth, resulting in a higher pitch.
This is why a violin is a higher pitch than a cello, the smaller volume of air inside the instrument is vibrating more quickly. Same goes for a guitar vs a ukulele.
It’s also why when you blow over the top of a bottle, the pitch gets lower as you drink more from it-there’s a bigger volume of air so it resonates more slowly.
So, as you’re filling a water bottle, the volume of air is getting smaller, making it vibrate more quickly, raising the pitch higher and higher as it fills.
I always thought it was because as you fill the bottle, there’s more water (soundproofing) and less air. Because there is less air, the less space for more lower sounds to form, thus leading to a higher pitch.
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