Why does the sound of filling up a water bottle change the higher up it gets?
In: Physics
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.
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